Sunan An-Nasa'i
Hadith #222
It 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