Musnad Ahmad 32:457

Hashim narrated to us, Sulayman narrated to us, from Thabit, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "I will come to the gate of Paradise on the Day of Resurrection and ask for it to be opened. The keeper will say: 'Who are you?' He said: So I will say: 'Muhammad.' He said: He will say: 'Regarding you, I was commanded not to open for anyone before you.'"