Musnad Ahmad 32:1347

'Arim narrated to us, Mu'tamir narrated to us, he said: I heard my father narrating from Anas, that the Prophet (peace and blessings of Allah be upon him) said: "Every prophet has asked a question," or he said: "For every prophet there is a supplication with which he has supplicated, and I have hidden my supplication as an intercession for my nation on the Day of Resurrection," or as he said.