Musnad Abu Ya'la al-Mawsili 115:324

Abu Musa narrated to us, Ibn Abi Adi and Abd al-A’la narrated to us, from Sa’id, from Qatadah, from Anas, that Umm Sulaym asked the Prophet of Allah (peace be upon him) about a woman who sees in her sleep what a man sees. The Prophet of Allah (peace be upon him) said: "If that happens in her sleep, then she must perform ghusl (ritual bath)." Umm Sulaym said: "I felt shy because of that, and I said: 'Does that happen?'" The Prophet of Allah (peace be upon him) said: "Yes. For from where would the resemblance come? Indeed, the fluid of the man is thick and white, and the fluid of the woman is yellow and thin. Whichever of them rises or precedes, the resemblance comes from it."