Musnad Ahmad 32:436

Bahz and 'Affan narrated to us, they said: Hammam narrated to us, Qatadah narrated to us, from Anas, that 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, and it was answered for him. And I have reserved my supplication as an intercession for my nation on the Day of Resurrection."