Genesis 26:5

My statutes: Things that the evil inclination and the nations of the world oppose, such as eating pig or wearing "mingled together" (sha'atnez), where there is no reason for the thing except the decree of the King and His statutes for His servants.

on account that Abraham obeyed My Word, and kept the keeping of My word, My statutes, My covenants, and My laws.

AND HE KEPT MY CHARGE. Rashi comments: “As a reward that Abraham hearkened to My voice when I tested him. And he kept My charge — these are the precautionary decrees instituted by the Sages, which are intended to make us avoid the violation of Biblical laws, such as Second Degrees of forbidden marriages and certain prohibited acts on the Sabbath. My commandments — these are precepts which, had they not been written in the Torah, were requisite to have been written, such as robbery and murder. My statutes — these are matters against which the evil inclination and the heathen nations argue, such as the prohibitions against eating the swine and the wearing of garments woven of wool and linen, there being no apparent rationale for them except that they are decrees of the King imposed on His subjects. And My laws — the plural is intended to include, [besides the Written Law], the Oral Law as well as those rules given to Moses from Sinai.” [Thus far the words of Rashi.]
Now if so, all this interpretation is posited on the opinion that Abraham fulfilled and observed the Torah before it was given on Sinai. This is indeed what the Sages of the Midrash said in connection with the verse, And Joseph gave them ‘agaloth’ (wagons),, 45:21. The word agaloth (wagons) may also mean “heifers,” thus suggesting that as a mark of identification to his father, Joseph gave his brothers a reference to the law of the Heifer (Deuteronomy 21:6) which he studied with his father just before he became separated from him. The Midrash referred to is in Bereshith Rabbah 95:2. thereby indicating to his father that when he left him they were studying the section dealing with Eglah Arufah. Thus Joseph occupied himself with Torah just as his fathers did. Though the Torah had not yet been given, it is still written of Abraham, And he kept My charge, My commandments, My statutes, and My laws. There the Sages also said that Abraham observed the details of the Torah, which he taught to his children, etc., etc. (Above, 18:19.) Bereshith Rabbah, ibid.
The question presents itself: If it be the case [that the laws of the Torah were observed by our ancestors before the Torah was given on Sinai], how did Jacob erect a pillar and marry two sisters in their lifetime, and, in the opinion of our Rabbis, four sisters. Also, Amram [Moses’ father] married his aunt, and Moses our teacher erected twelve pillars., 24:4. …

בקולי , a reference to the binding of Yitzchok where the Torah had used the expression (22,18) עקב אשר שמעת בקולי, “as a consequence of your heeding My instructions.

וישמור משמרתי, a reference to the circumcision, where G’d had used this expression (את בריתי תשמור) describing the command in 17,9 to circumcise himself.

מצותי, the command to perform circumcision when the baby is eight days old; as in 21,4.

חוקותי ותורותי, basically these include all the laws known to mankind prior to the giving of the Torah, such as not murdering, not stealing, etc. They include virtues such as extending hospitality. At the time the Torah was revealed, these were renewed and elaborated on. At that time the covenant to keep these laws forever was concluded with the Jewish people.

עקב אשר שמע אברהם בקולי, in accordance with all that I commanded him;

וישמור משמרתי, he also did the very things which I Myself am in the habit of doing, such as to perform deeds of loving kindness. (compare Psalms 25,10 כל ארחות ה' חסד ואמת, “all the paths of the Lord are love and truth, etc.”) Also, Avraham warned the potential sinners to improve their ways, something that is of utmost importance to Me. This is what he did every time the Torah records him as קרא בשם ה', “he proclaimed the name of the Lord.” He also observed My revealed commandments מצותי, חוקותי, ותורותי, “the seven laws I laid down for all of mankind.” As a result of all this, he did not only preach to his fellow man but he practiced what he preached. Due to his possessing this virtue he became a model human being for others to emulate. At this particular stage in Yitzchok’s life, G’d did favours to him which were the result of the merits of others, i.e. the merit of his father. This theme is repeated once more in verse 24 when the Torah attributes the promise of the increase in Yitzchok’s seed to the merit of Avraham, i.e.בעבור אברהם עבדי “for the sake of My servant Avraham.” Neither Yaakov nor Avraham had ever been told that G’d’s promises to them was on account of a third party’s merit. The reason G’d’s promises to Yitzchok had been due to other people’s merits was that at that time he had not yet emulated his father’s practice of proclaiming the name of G’d, i.e. calling his fellow man to improve their lifestyles and to abandon idolatry. Once Yitzchok had begun to do this we find that even an Avimelech is in awe of him and recognises that he is the recipient of G’d’s blessings in his own right. (compare verse 26, as well as verses 28-29. All this was due to the fact that in verse 25 he is reported as emulating his father, building an altar and proclaiming the name of the Lord. Yaakov, as opposed to his father Yitzchok, has been described as being a יושב אהלים from his earliest youth, i.e. devoting himself to promoting matters spiritual, studying as well as teaching in the academy of Shem and Ever, (primarily those of the seven Noachide commandments which regulate inter personal relations.)

עקב אשר שמע אברהם, the reason why I have given him the land is because he heeded my voice even to the extent of offering his only son as a burnt-offering.

משמרתי, מצותי, חוקותי, ותורותי, G’d included all the commandments, hinting that all the commandments, i.e. chukotai, contain elements of which the rationale has been revealed and some which have not. Even the seven Noachide laws, applicable universally, contain elements the reason for which has not ever been revealed to the people but whose rationale is familiar only to the wisest scholars. Examples of such commandments are: the prohibition of grafting or crossbreeding, eating (amputated) limbs of a living animal אבר מן החי. To describe such laws, the Torah here mentions the word חוקותי, “My statutes,” whereas the description מצותי refers to commandments which the human mind understands or even applauds. It matters not that these commandments be performed by different parts of the body, i.e. the mouth, the hands, or merely the heart. Similarly, it matters not whether they are positive commandments, requiring action, or negative commandments requiring self-restraint. Some commentators (compare Torah Shleymah 25) claim that Avraham was only 3 years old when he recognised the existence of G’d and began to worship him, so that the numerical value of the letters in the word עקב=172 would comprise the number of years which Avraham carried out G’d’s commandments. Other commentators place the time when Avraham became convinced of monotheism as the only religion as being when he had completed his fortieth year. (Bereshit Rabbah 30,8) This appears to be more probable. Our sages claim that Avraham observed all the commandments spelled out in the written and the oral Torah including such rabbinic ordinances as Eyruv Tavshilim (Yuma 28). They base this on the word משמרתי, seeing this word refers generally to the type of “security fence” סיג, introduced by the Rabbis to safeguard us against violating Biblical injunctions. What the rabbis meant therefore was that Avraham invented such safeguards for himself in order not to violate the basic prohibitions.