Sayyidah Umm Sulaym (may Allah be pleased with her) related that she asked the Messenger of Allah (peace and blessings be upon him) about a woman who sees in her sleep what a man sees. The Messenger of Allah (peace and blessings be upon him) said: "When a woman sees that, she should perform ghusl." Umm Salamah (may Allah be pleased with her) says: Feeling shy, I asked: Does this really happen? The Messenger of Allah (peace and blessings be upon him) said: "Then where does the resemblance come from? The man's water is thick and white, and the woman's water is thin and yellow; whichever of the two prevails or comes first, the resemblance comes from it."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 797