سنن أبي داود

Narrated Ikrimah:

A man made his wife like the back of his mother. He then had intercourse with her before he atoned for it. He came to the Prophet (ﷺ) and informed him of this matter. He asked (him): What moved you to the action you have committed? He replied: I saw the whiteness of her shins in moon light. He said: Keep away from her until you expiate for your deed.