Sunan an-Nasa'i 222
سنن النسائي#222SahihIt was narrated from Ghudaif bin Al-Harith that he asked 'Aishah in which part of the night would the Messenger of Allah (ﷺ) perform Ghusl? She said:"Sometimes he performed Ghusl at the beginning of the night and sometimes he performed Ghusl at the end." I said: "Praise be to Allah Who has made the matter flexible
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Abu GhuddahSahih
Al-AlbaniSahih
Zubair Ali ZaiHasan
In-book reference: 1:223