Musnad Abu Ya'la al-Mawsili 115:1

Hadb ibn Khalid narrated to us, Hammam narrated to us, Qatadah narrated to us, from Anas, from the Prophet (may Allah bless him and grant him peace), who said: "Indeed, for every prophet there is a supplication with which he supplicated and it was answered for him. And I have hidden my supplication as an intercession for my nation."