Musnad Abu Ya'la al-Mawsili 115:87

Ubayd Allah ibn Umar narrated to us, Harami ibn Umarah narrated to us, Shu'bah narrated to us, from Qatadah, from Anas, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "For every prophet there is a supplication with which he supplicated, and I have saved my supplication as an intercession for my nation on the Day of Resurrection."