Sunan an-Nasa'i, Book 1, Hadith 266 (In-Book) ⓘ
It was narrated from Anas that the Messenger of Allah used to (go around) all his wives and perform Ghusl once.
أَخْبَرَنَا مُحَمَّدُ بْنُ عُبَيْدٍ، قَالَ حَدَّثَنَا عَبْدُ اللَّهِ بْنُ الْمُبَارَكِ، قَالَ أَنْبَأَنَا مَعْمَرٌ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم كَانَ يَطُوفُ عَلَى نِسَائِهِ فِي غُسْلٍ وَاحِدٍ .
also referenced under: Volume 1, Book 1, Hadith 265 (DarusSalam)