Musnad Ahmad 1153:595

Qutaybah narrated to us, he said: Ibn Lahi'ah narrated to us, from Abu al-Aswad, from 'Urwah, from 'Aishah, she said: When the Messenger of Allah (peace and blessings of Allah be upon him) was in a state of major ritual impurity (janabah) and wanted to sleep while in that state of impurity, he would perform his ablution for prayer before sleeping. And he used to say: "Whoever wants to sleep while in a state of major ritual impurity, let him perform his ablution for prayer."