Musnad Abu Ya'la al-Mawsili 115:275

Al-Abbas ibn al-Walid narrated to us, Yazid narrated to us, Sa'id narrated to us, from Qatadah, that Anas narrated to them that Umm Sulaym narrated that she asked the Prophet, may Allah bless him and grant him peace, about a woman who sees in her sleep what a man sees. He said: "If a woman sees that, then let her perform ghusl (ritual bath)." Umm Sulaym said: "I felt shy because of that, and I said: 'Does this happen?'" The Prophet of Allah, may Allah bless him and grant him peace, said: "Yes. For from where would the resemblance come? Indeed, the man's fluid is thick, and the woman's fluid is thin and yellow. So whichever of them prevails or precedes, the resemblance comes from it."