It is narrated from Hazrat Aisha (may Allah be pleased with her) that when the Prophet (peace and blessings be upon him) wanted to sleep or eat in a state requiring ghusl, he would perform ablution as for prayer.
Hadith Referenceمسند احمد / مسند النساء / 26342
Hadith Gradingحکم دارالسلام:حديث صحيح، محمد بن إسحاق صرح بالتحديث، خ: 286، م: 305