It is narrated from Hazrat Aisha (may Allah be pleased with her) that when the Prophet (peace and blessings be upon him) wished to sleep in a state requiring ghusl, he would perform ablution as for prayer.
Hadith Referenceمسند احمد / مسند النساء / 24969
Hadith Gradingحکم دارالسلام:حديث صحيح، عبدالوهاب إن كان فيه كلام ولكن متابع