Musnad Ahmad 5:196

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, from Mansur, from Rib'i ibn Hirash, from Ali, from the Prophet, may Allah bless him and grant him peace, that he said: "A servant does not believe until he believes in four things: until he testifies that there is no god but Allah, and that I am the Messenger of Allah, whom He sent with the truth; and until he believes in the resurrection after death; and until he believes in the decree (al-qadar)."