Deuteronomy 8:1

If you have begun a commandment, be sure to finish it, for only the one who finishes it get the credit, as it is written (Joshua 24:32), "The children of Israel buried the bones of Joseph, which they had brought up from Egyp, in Sh'khem." But did not Moses alone bring the bones up (from Egypt)? Rather, (this teaches us that) because he did not manage to finish the commandment (of burying Joseph), the children of Israel did it and they received the credit.

Every commandment which I command you this day, observe ye to do, that you may live, and multiply, and go in and inherit the land which the Lord sware to your fathers.

every commandment That is to say: If you desire to observe the commandments — so that you may live — then

ALL THE COMMANDMENT. The purport of it is to state: “Although I have admonished you [specifically] with respect to the ordinances, all the [other] commandments shall ye observe to do also, for in observing all of them [the ordinances and the commandments] you will live and be fruitful in the fruit of the body, the fruit of the cattle, and the fruit of the ground,” all as mentioned above., Verse 13.

כל המצוה אשר אנכי מצוך היום תשמרון לעשות, the objective of people worshipping idols is nothing other than their obsession with transient values which consist mostly of three categories, having children, living to a ripe old age, and success in amassing money or its equivalent. The Torah promises that by observing G’d’s commandments as spelled out in the Torah one may achieve all these three objectives. This is meant by the Torah writing: למען תחיון, when commanding the laws of the Torah G’d did not only mean to guarantee you eternal life in the hereafter, but He had in mind also successful, rewarding life on earth.

ורביתם, you will multiply by having numerous children.

ובאתם וירשתם, attaining wealth and honour by inheriting the land.

כל המצוה, “All the instruction, etc.” our author understands the word כל in our verse not so much as “all,” i.e. each and every, but as “the whole commandment.” Once you have begun to fulfill a commandment you must complete what you have undertaken. This rule has been spelled out specifically in the Jerusalem Talmud, tractate Rosh Hashanah, chapter 1, halachah 8. The text is: when someone has commenced to perform a commandment he is told to complete it. If someone has commenced to perform a commandment and it is completed by someone else, the last person gets the credit for having performed it. The source of this ruling is traced to Moses who had commenced the commandment of transferring Joseph’s remains to the land of Israel, (Exodus 13,19) but who could not complete it as he never crossed the Jordan. The credit for burying his remains in the land of Israel is therefore given to the Jewish people, as we know from Joshua 24,32: “and the Children of Israel buried Joseph’s remains which they had brought with them from Egypt, and they were interred in the city of Sh’chem on a plot of land which his father Yaakov had acquired for the price of 1000 kessito as spelled out in Genesis 37,27. This plot became an ancestral heritage to the tribe of Joseph.” (Compare Babylonian Talmud, tractate Sotah) folio 13. Rabbi Sh’muel bar Nachmani is quoted in the Talmud, tractate Sotah folio 13, as saying that if someone commences to fulfill a commandment but fails to complete it (not by circumstances beyond his control), he will wind up burying his wife and his children. He adds that we know this from Yehudah (Yaakov’s son) compare Genesis 37,26, who said: “what profit is there in killing our brother Joseph.” Yehudah, of course buried both his wife and two of his sons, as we read in Genesis 38, 7-12. Some scholars add that this was why the brothers demoted him as we know from the beginning of that chapter. Instead of saving Joseph, the brothers sat down to eat their meal, and in the interval Joseph was taken out of the pit by Ishmaelites and sold as a slave. Yehudah said that they could not pronounce benedictions of G–d while planning to commit murder of their brother. They therefore interrupted, and sold Joseph, before they sat down again to eat. Yehudah is perceived as having begun to save Joseph from death but not having brought him back safely to his father. The Torah had testified that the brothers listened to him, so that it could be assumed that they would also have agreed to bring him back to his father. At any rate, this is the meaning of Moses referring to כל מצוה, “the whole commandment.”