Musnad Ahmad 32:1112

'Abd al-A'la narrated to us, Sa'id narrated to us, from Qatadah, from Anas ibn Malik, that Umm Sulaym asked the Prophet, peace and blessings of Allah be upon him, saying: "Does a woman see what a man sees in her sleep?" The Prophet of Allah, peace and blessings of Allah be upon him, said: "If she sees what a man sees—meaning the fluid—then she must perform ghusl (ritual bath)." Umm Salamah said: "Does that happen?" The Prophet of Allah, peace and blessings of Allah be upon him, said: "Yes. The man's fluid is thick and white, and the woman's fluid is thin and yellow. Whichever of them precedes or prevails—Sa'id said: 'We are in doubt'—the resemblance comes from it."