It was narrated that 'Ali said:
"The Messenger of Allah (ﷺ) said: 'No slave truly believes until he believes in four things: in Allah alone with no partner; that I am the Messenger of Allah; in the resurrection after death; and in the Divine Decree (Qadar).'"
English reference: Vol. 1, Book 1, Hadith 81
Arabic reference: Book 1, Hadith 85