Genesis 12:11

And it was, as he approached to enter the limit of Mizraim, and they had come to the river, and were uncovering their flesh to pass over, that Abram, said to Sara his wife, Behold, until this I have not beheld thy flesh; but now I know that thou art a woman of fair aspect.

Behold I now realize. In Canaan there were other women as beautiful as Sarah but not in Egypt, because physical appearance is affected by climate

BEHOLD NOW, I KNOW, ETC. 13. SAY, I PRAY, THOU ART MY SISTER. I do not know why Abraham was more fearful for her now than before. And if we say, as Rashi explains, that it was because the Egyptians were black and repulsive, now, to Abimelech, king of the Philistines, he also said so, he as well as Isaac,, 26:7. who lived in that land by command of G-d., Verse 3. Perhaps the Canaanites in that generation were steeped in idolatry but restrained from unchastity more than the Egyptians and the Philistines. But this is not correct. It is possible that Abraham and Sarah had no fear until they came into a royal city for it was their custom to bring the king a very beautiful woman and to slay her husband through some charge they would contrive against him.
It appears to me correct that such was their procedure from the time they left Haran. At every place he would say, “She is my sister,” for so Abraham said, And it came to pass, when G-d caused me to wander from my father’s house, etc., 20:13. Scripture, however, mentions it only concerning those places where something happened to them on account of it. Thus Abraham now alerted Sarah as he had charged her from the beginning. Isaac, on the other hand, was not afraid in his country and in his city. Only when he came to the land of the Philistines did he adopt his father’s way.
He [Abraham] said, That it may be well with me for thy sake, and that my soul live because of thee, meaning “as long as we are strangers in this land, until the famine will pass,” for Abraham came to live in the land of Egypt on account of the famine. When the famine passes, he would return to the land concerning which he had been commanded and which G-d had given to him and his children. He thus thought that they [he and his wife] would live through the famine and that relief and succor will come to them from G-d enabling them to return, or that it may be possible for them to flee to the land of Canaan when they will give them up.
Now Rashi wrote, “ ‘Hinei na’ (Behold now), I know. A Midrash Agada: Until now he had not perceived her beauty on account of her modesty. Now, however, [he became cognizant of it] through an event. Another interpretation is that because of the exertion of travelling a person usually becomes uncomely, but she [Sarah] has retained her beauty. Still the plain sense of the text is this: ‘Behold, now the time has come to be anxious because of your beauty. I have long known that …

כאשר הקריב, close to the time when it became necessary to take such precautions, so that he would not forget.

לבא מצרימה, a country which was equivalent to a giant brothel. This is mentioned even more forcefully in Ezekiel 23,20 where the prophet describes the carnal attitudes of the Egyptians as אשר בשר חמורים בשרם, וזרמת סוסים זרמתם, “those whose flesh is the flesh of donkeys, and whose sperm is the sperm of horses.”

ויהי כאשר הקריב, not just he himself, but when he had brought his camp close to the Egyptian border. He was traveling slowly, in keeping with the pace of those animals that could not move quickly. Seeing that the word הקריב is in a transitive mode, the verse must refer to Avram’s entire entourage. We encounter the same construction when the Torah reports the pursuit of the Israelites by Pharaoh in Exodus 14,10, i.e. ויהי כאשר הקריב פרעה, when the reference is to Pharaoh and his army.

הנה נא ידעתי, even though the word נא generally describes a knowledge which one has just acquired, Avram did not want Sarai to think that he had only now discovered her good looks. He therefore used a formulation which indicated that he had been aware of her beauty ever since he had first met her. Similar constructions without the addition of ידעתי in the past tense are found in 19,2 הנה נא אדני, “now my lords,” or in 19,19 הנה נא מצא עבדך , חן, “now that your servant has found favour.” The comparison with these examples shows that the addition of the word ידעתי in our example had as its purpose to reassure Sarai that he had not only suddenly discovered that she was physically attractive.

הנה נא ידעתי כי אשה יפת מראה את, “I am well aware that you are a physically attractive woman;” according to the interpretation of Rabbi Joseph Kara, the Torah abbreviated what Avraham said to Sarah, the full sentence including what is reported here as a separate sentence: “I know that because you are a beautiful –white skinned – woman as soon as they see you they will kill me and take you to the king’s palace to be part of his harem. In order to save my life, say that you are my sister, instead of my wife.” Although the Egyptians were idolaters, they observed the law of not sleeping with someone else’s wife. As a result, they resorted to murder instead. This sounds strange, as the same lawgiver that prohibited adultery also prohibited murder. Their reasoning was-presumably, that it is better to murder and commit that sin once, than to commit the sin of adultery day after day. Besides, they were afraid that any husband, whose wife had been kidnapped, would complain to their respective kings about this. Once the original husband was dead, no restitution could be made.