Genesis 9:21-23

21He drank some of the wine and became drunk, and he was uncovered and lay exposed inside his tent.

22Ham, the father of Canaan, saw [by accident] the nakedness of his father, and [to his father’s shame] told his two brothers outside.

23So Shem and Japheth took a robe and put it on both their shoulders, and walked backwards and covered the nakedness of their father; their faces were turned away so that they did not see their father’s nakedness.