Sahih Muslim Book 1, Hadith Number 391.

Chapter : Pertaining to the words of the Apostle of Allah (may peace be upon him): “I would be the first among people to Intercede in the Paradise and among the Apostles I would have the largest following”.

Abu Huraira reported: The Messenger of Allah (may peace be upon him) said: There was for every apostle a prayer with which he prayed for his Ummah and it was granted to him; but I wish, if Allah so wills, to defer my prayer for the intercession of my Ummah on the Day of Resurrection.