Sunan an-Nasa'i, Book 14, Hadith 1389 (In-Book) ⓘ
It was narrated that Jabir said: "The Messenger of Allah said: 'Every Muslim man has to perform Ghusl one day in every seven, and that is on Friday.'"
أَخْبَرَنَا حُمَيْدُ بْنُ مَسْعَدَةَ، قَالَ حَدَّثَنَا بِشْرٌ، قَالَ حَدَّثَنَا دَاوُدُ بْنُ أَبِي هِنْدٍ، عَنْ أَبِي الزُّبَيْرِ، عَنْ جَابِرٍ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " عَلَى كُلِّ رَجُلٍ مُسْلِمٍ فِي كُلِّ سَبْعَةِ أَيَّامٍ غُسْلُ يَوْمٍ وَهُوَ يَوْمُ الْجُمُعَةِ " .
also referenced under: Volume 2, Book 14, Hadith 1379 (DarusSalam)