Lady Aisha (may Allah be pleased with her) narrates that when the Messenger of Allah (peace and blessings be upon him) intended to sleep in a state of major ritual impurity (janabah), he would perform ablution (wudu) as he did for prayer.
Hadith Referenceصحيح ابن خزيمه / جماع أبواب فضول التطهير والاستحباب من غير إيجاب / 213