Sahih Muslim
The Book of Faith (Kitab Al-Iman)
Book Number: 001 / Hadith Number: 386
Abu Huraira reported: The Messenger of Allah (may peace be upon him) said: There is for every apostle a prayer, and I intend (if Allah so willed) that I would reserve my prayer for the intercession of my Ummah on the Day of Resurrection.