Musannaf Ibn Abi Shayba 1:879

Yazid ibn Harun narrated to us, he said: Sa'id ibn Abi 'Aruba narrated to us, from Qatada, from Anas, that Umm Sulaym asked the Messenger of Allah (may Allah bless him and grant him peace) about a woman who sees in her sleep what a man sees. The Messenger of Allah (may Allah bless him and grant him peace) said: "If she sees that and she discharges, then upon her is the ritual bath (ghusl)." Umm Salama said: "O Messenger of Allah, does this happen?" He said: "Yes. The man's fluid is thick and white, and the woman's fluid is thin and yellow. So whichever of them precedes or rises above the other, the child will resemble it."