Sahih Muslim, Chapter: The Book of Menstruation (Kitab Al-Haid)

Umm Sulaim narrated it that she asked the Apostle of Allah (may peace be upon him) about a woman who sees in a dream what a man sees (sexual dream). The Messenger of Allah (may peace be upon him) said: In case a woman sees that, she must take a bath. Umm Sulaim said: I was bashful on account of that and said: Does it happen? Upon this the Apostle of Allah (may peace be upon him) said: Yes (it does happen), otherwise how can (a child) resemble her? Man's discharge (i. e. sperm) is thick and white and the discharge of woman is thin and yellow; so the resemblance comes from the one whose genes prevail or dominate.