Musannaf Ibn Abi Shayba 21:784

Yahya ibn Abi Bukayr narrated to us, he said: Isra'il narrated to us, he said: Yusuf ibn Abi Burdah informed us, he said: I heard my father say: I entered upon 'Aisha and I heard her saying: When the Messenger of Allah (peace and blessings of Allah be upon him) would exit from the privy (al-gha'it), he would say: "Your forgiveness (ghufranaka)."