Musnad Ahmad 1153:1739

Waki’ narrated to us, from Nafi’—meaning Ibn ‘Umar—from Salih ibn Sa’id, from ‘Aisha, that she missed the Prophet, may Allah bless him and grant him peace, from his bed. She touched him with her hand, and it fell upon him while he was prostrating, and he was saying: "O Lord, give my soul its piety, and purify it; You are the best of those who purify it. You are its Guardian and its Master."