Sayyiduna Adi bin Hatim (may Allah be pleased with him) said: I asked the Messenger of Allah (peace and blessings be upon him) about a person in a state of major impurity (junub) who goes to sleep (what should he do)? He (peace and blessings be upon him) said: "He should perform ablution like his ablution for prayer."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 14325