Musnad Ahmad 87:5

Wahb ibn Jarir narrated to us, my father narrated to us, he said: I heard Mansur ibn Zadhan narrating from Maymun ibn Abi Shabib, from Qays ibn Sa’d ibn ‘Ubadah, that his father sent him to the Prophet, peace and blessings of Allah be upon him, to serve him. He said: I came to the Prophet, peace and blessings of Allah be upon him, and I had prayed two rak'ahs. He said: So he struck me with his foot and said: "Shall I not guide you to a door from the doors of Paradise?" I said: "Yes." He said: "There is no power and no strength except by Allah (La hawla wa la quwwata illa billah)."