Musnad Ahmad 32:1227

Rawh narrated to us, Shu'bah narrated to us, from Qatadah, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, for every prophet there is a supplication with which he has supplicated for his nation, and I have reserved my supplication as an intercession for my nation."