When Abraham went with Sarah to Egypt, he said: "Now I know that you are a beautiful woman." Why did this happen on their way to Egypt? I heard that this happened specifically there because Abraham was negatively influenced by Egypt's Tumah. When they got there, his high spiritual level suffered a decline and he got a little more connected to the material world, noticing materialistic details he didn't notice until then, such as Sarah's beauty. Does anyone know where this is written? I heard it in the name of Arizal, but I'm not sure...
Similary, the Midrash Tanchuma (Lech Lecha 5) describes how the behaviour of Avraham changed when he entered Mitzrayim:
When they reached the Egyptian border, and were standing on the bank of the Nile, our patriarch Abraham noticed that Sarah’s reflection in the river was like the radiance of the sun. From this statement, we may conclude that he had not previously been aware of her beauty. (emphasis mine)
The Rabbeinu Bahya on Bereishis 12:11 builds on this Midrash and says:
Avraham had never looked at Sarai in a way that made him notice her beauty as he had made a promise to his eyes not to use them in a manner which would arouse his desire for physical gratification.
In Yesod HaBayis: Building Foundations for Life, written by Rabbi Chaim Dov Stark, it is explained (Chapter 5: Surviving and thriving in an Unfiltered World, p. 52-53):
The meforshim tell us that even the air of Mitzrayim was permeated by tumah. Simply entering the country was enough to cause even the greatest tzadikkim to be in danger of spiritual harm. The Torah tells us that when Avraham Avinu entered Mitzrayim, he became aware of Sarah's external beauty for the first time.
Rabbi Chaim Dov Stark then goes on to bring an Arizal, as quoted by the author of Nesivos Hamishpat:
The Arizal, as quoted by the Nesivos Hamishpat, states that Avraham Avinu was on such a lofty level of kedushah that, under normal circumstances, he was incapable of even seeing Sarah's external appearance. Once he entered Mitzrayim, however, the impurity of the land itself lowered his level of kedushah to the extent that he was able to perceive her phyiscal beauty.
In the footnotes in Yesod HaBayis (p. 53, footnote 4) it explains this idea further. It is in Hebrew, so I tried to translate it. It basically says that Avraham Avinu was like Adam HaRishon, before the "original sin", where Adam and Chava noticed each others nakedness. Avraham Avinu was on that same level as Adam. But, when Avraham Avinu entered Mitzrayim, he was lowered from this level, and was now able to see the physical beauty of his wife Sarah.
Rabbi David Hanania Pinto explains in Torat David (p. 181):
Even in their private life, they refrained from immoderation, according to their high spiritual level, and denied themselves any overindulgence of the senses. This is why Avraham was not overly familiar with his wife’s beauty.
This seems to accept the idea that before entering Mitzrayim, Avraham Avinu had such a high spiritual level, that he never looked at Sarah the way he did when entering Mitzrayim.
My own chiddush is that Avraham Avinu "went down to Mitzrayim". Going down often symbolizes going the other way from G-d, further away from G-d. That's why G-d said to Avraham: Lech lecha, meaning to come closer to G-d. See this related commentary of the Toldos Yaakov Yosef on parashas Bo.
I found where this is written. It was Baal Shem Tov who wrote this explanation.