(7) باب وُجُوبِ الْغُسْلِ عَلَى الْمَرْأَةِ بِخُرُوجِ الْمَنِيِّ مِنْهَا
Anas b. Malik reported that Umm Sulaim narrated it that she asked the Messenger of Allah (ﷺ) about a woman who sees in a dream what a man sees (sexual dream). The Messenger of Allah (may peace be upon bi m) 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 Messenger of Allah (ﷺ) 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.
Reference: Sahih Muslim 311
In-book reference: Book 3, Hadith 32
USC-MSA web (English) reference(deprecated numbering scheme): Book 3, Hadith 608