Musnad Ahmad 32:2066

Muhammad ibn Ja‘far narrated to us, Sa‘id narrated to us, from Qatadah, from Anas ibn Malik, that his mother, Umm Sulaym, asked the Messenger of Allah (may Allah bless him and grant him peace) and said: "A woman sees in her dream what a man sees." He said: "If she sees that in her dream, then she must perform ghusl (ritual bath)." Umm Salamah, the wife of the Prophet (may Allah bless him and grant him peace), said—and she was shy—: "Does this happen, O Messenger of Allah?" He said: "Yes. For from where would the resemblance come? The man’s fluid is white and thick, and the woman’s fluid is yellow and thin. Whichever of them precedes—or prevails—the resemblance comes from it."