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. (Using translation from Muslim 311)
أَنَّ أُمَّ سُلَيْمٍ سَأَلَتِ النَّبِيَّ ﷺ عَنِ الْمَرْأَةِ تَرَى فِي مَنَامِهَا مَا يَرَى الرَّجُلُ فَقَالَ النَّبِيُّ ﷺ «إِذَا رَأَتِ الْمَاءَ فَلْتَغْتَسِلْ» قَالَتْ أُمُّ سَلَمَةَ وَاسْتَحْيَيْتُ مِنْ ذَلِكَ وَهَلْ يَكُونُ ذَلِكَ يَا رَسُولَ اللهِ؟ قَالَ «نَعَمْ إِنَّ مَاءَ الرَّجُلِ غَلِيظٌ أَبْيَضُ وَمَاءُ الْمَرْأَةِ رَقِيقٌ أَصْفَرُ فَمِنْ أَيِّهِمَا عَلَا أَوْ سَبَقَ كَانَ مِنْهُ الشَّبَهُ»