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."
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا عَمْرُو بْنُ هِشَامٍ، قَالَ حَدَّثَنَا مَخْلَدٌ، عَنْ سُفْيَانَ، عَنْ أَبِي الْعَلاَءِ، عَنْ عُبَادَةَ بْنِ نُسَىٍّ، عَنْ غُضَيْفِ بْنِ الْحَارِثِ، أَنَّهُ سَأَلَ عَائِشَةَ - رضى الله عنها - أَىُّ اللَّيْلِ كَانَ يَغْتَسِلُ رَسُولُ اللَّهِ صلى الله عليه وسلم قَالَتْ رُبَّمَا اغْتَسَلَ أَوَّلَ اللَّيْلِ وَرُبَّمَا اغْتَسَلَ آخِرَهُ . قُلْتُ الْحَمْدُ لِلَّهِ الَّذِي جَعَلَ فِي الأَمْرِ سَعَةً .
sunnah.com