Musnad Ahmad 2:110

Waki’ narrated to us, Kahmas narrated to us, from Ibn Buraydah, from Yahya ibn Ya’mar, from Ibn ‘Umar, from ‘Umar: That Gabriel, peace be upon him, said to the Prophet, may Allah bless him and grant him peace: "What is faith?" He said: "To believe in Allah, His angels, His books, His messengers, the Last Day, and in the decree, its good and its evil." Gabriel said to him: "You have spoken the truth." He said: So we were amazed at him; he asks him and he confirms him. He said: Then the Prophet, may Allah bless him and grant him peace, said: "That was Gabriel; he came to you to teach you the landmarks of your religion."