As Avram and Sarai approach Egypt, Avram tells her that she is a beautiful woman. They devise a plan to tell the Egyptians that they are brother and sister.

Sources:

Gemara Sotah 17 A

Parshas Lech Lecha Perek Perek 10 Pessukim 10-13

Rashi Possuk 11

Taamei Masores Hatorah Maharam MeRotenberg Possuk 12

Gemara Brochas 31 B