Sahih Muslim 311
Sahih Muslim · Book 3: The Book of Menstruation · Women are obliged to perform ghusl if they emit fluid
Anas b. Malik reported that Umm Sulaim narrated it that she asked the Messenger of Allah (ﷺ) about a woman who sees in a dream what a man sees (sexual dream). The Messenger of Allah (may peace be upon bi m) said:
حَدَّثَنَا عَبَّاسُ بْنُ الْوَلِيدِ ، حَدَّثَنَا يَزِيدُ بْنُ زُرَيْعٍ ، حَدَّثَنَا سَعِيدٌ ، عَنْ قَتَادَةَ ، أَنَّ أَنَسَ بْنَ مَالِكٍ ، حَدَّثَهُمْ أَنَّ أُمَّ سُلَيْمٍ حَدَّثَتْ أَنَّهَا، سَأَلَتْ نَبِيَّ اللَّهِ صلى الله عليه وسلم عَنِ الْمَرْأَةِ تَرَى فِي مَنَامِهَا مَا يَرَى الرَّجُلُ فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِذَا رَأَتْ ذَلِكِ الْمَرْأَةُ فَلْتَغْتَسِلْ " . فَقَالَتْ أُمُّ سُلَيْمٍ وَاسْتَحْيَيْتُ مِنْ ذَلِكَ قَالَتْ وَهَلْ يَكُونُ هَذَا فَقَالَ نَبِيُّ اللَّهِ صلى الله عليه وسلم " نَعَمْ فَمِنْ أَيْنَ يَكُونُ الشَّبَهُ إِنَّ مَاءَ الرَّجُلِ غَلِيظٌ أَبْيَضُ وَمَاءَ الْمَرْأَةِ رَقِيقٌ أَصْفَرُ فَمِنْ أَيِّهِمَا عَلاَ أَوْ سَبَقَ يَكُونُ مِنْهُ الشَّبَهُ " .
In case a woman sees that, she must take a bath. Umm Sulaim said: I was bashful on account of that and said: Does it happen? Upon this the Messenger of Allah (ﷺ) said: Yes (it does happen), otherwise how can (a child) resemble her? Man's discharge (i. e. sperm) is thick and white and the discharge of woman is thin and yellow; so the resemblance comes from the one whose genes prevail or dominate.