Musnad Abu Ya'la al-Mawsili 115:256

Zuhayr narrated to us, Affan narrated to us, Hammam narrated to us, from Qatadah, from Anas, that the Prophet (peace and blessings of Allah be upon him) said: "Indeed, for every prophet there is a supplication with which he supplicated and it was answered for him. And I have reserved my supplication as an intercession for my nation on the Day of Resurrection."