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."
English reference: Vol. 1, Book 1, Hadith 223
Arabic reference: Book 1, Hadith 224