(Genesis 12:11-13) "And it came to pass, when he drew near to enter Egypt, etc." All these years she was with him, and only now does he say to her, "Behold now, I know [that you are a woman fair to look upon]"? Rather, [it was] because through the journey a person becomes disfigured [from the hardship of travel, yet she remained beautiful]. Another interpretation: We traveled through Aram Naharaim and Aram Nahor, and we did not find a woman as beautiful as you; and now we are entering a place of ugly and dark people [and your beauty will draw dangerous attention].
Abraham Tells Sarah She Is a Beautiful Woman
Yalkut Shimoni on Torah 67:5
(בראשית יב יא-יג) וַיְּהִי כַּאֲשֶׁר הִקְרִיב לָבוֹא מִצְרָיְמָה וְגוֹ' כָּל הַשָּׁנִים הַלָּלוּ הִיא עִמּוֹ, וְעַכְשָׁיו אוֹמֵר לָהּ, הִנֵּה נָא יָדַעְתִּי אֶלָּא שֶׁעַל יְדֵי הַדֶּרֶךְ אָדָם מִתְבַּזֶּה דָּבָר אַחֵר, הָלַכְנוּ בְּאֲרַם נַהֲרַיִם וּבְאֲרַם נָחוֹר וְלֹא מָצָאנוּ אִשָּׁה יָפָה כְּמוֹתֵךְ, וְעַכְשָׁיו אָנוּ נִכְנָסִין לִמְקוֹם כְּעוּרִים וּשְׁחוֹרִים.