Sahih Muslim 711
The Book of Menstruation
حَدَّثَنَا دَاوُدُ بْنُ رُشَيْدٍ، حَدَّثَنَا صَالِحُ بْنُ عُمَرَ، حَدَّثَنَا أَبُو مَالِكٍ الأَشْجَعِيُّ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ سَأَلَتِ امْرَأَةٌ رَسُولَ اللَّهِ صلى الله عليه وسلم عَنِ الْمَرْأَةِ تَرَى فِي مَنَامِهَا مَا يَرَى الرَّجُلُ فِي مَنَامِهِ فَقَالَ " إِذَا كَانَ مِنْهَا مَا يَكُونُ مِنَ الرَّجُلِ فَلْتَغْتَسِلْ " .
Anas b. Malik reported:A woman asked the Messenger of Allah (way peace be upon him) about a woman who sees in her dream what a man sees in his dream (sexual dream). He (the Holy Prophet) said: If she experiences what a man experiences, she should take a bath
Text from the public-domain hadith-api dataset. Numbering follows that dataset and can differ by a few from some printed editions — check the wording, not only the number.