Sunan an-Nasa'i, Book 1, Hadith 199 (In-Book) ⓘ
It was narrated that Khawlah bin Hakim said: "I asked the Messenger of Allah about a woman who has a wet dream and he said: 'If she sees water, let her perform Ghusl.'"
أَخْبَرَنَا يُوسُفُ بْنُ سَعِيدٍ، قَالَ حَدَّثَنَا حَجَّاجٌ، عَنْ شُعْبَةَ، قَالَ سَمِعْتُ عَطَاءً الْخُرَاسَانِيَّ، عَنْ سَعِيدِ بْنِ الْمُسَيَّبِ، عَنْ خَوْلَةَ بِنْتِ حَكِيمٍ، قَالَتْ سَأَلْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم عَنِ الْمَرْأَةِ تَحْتَلِمُ فِي مَنَامِهَا فَقَالَ " إِذَا رَأَتِ الْمَاءَ فَلْتَغْتَسِلْ " .
also referenced under: Volume 1, Book 1, Hadith 198 (DarusSalam)