Sahih Muslim, Chapter: Faith (Kitab Al Iman)

that when it was the day of Khaibar a party of Companions of the Apostle (may peace be upon him) came there and said: So and so is a martyr, till they happened to pass by a man and said: So and so is a martyr. Upon this the Messenger of Allah remarked: Nay, not so verily I have seen him in the Fire for the garment or cloak that he had stolen from the booty, Then the Messenger of Allah (may peace be upon him) said: Umar son of Khattab, go and announce to the people that none but the believers shall enter Paradise. He ('Umar b. Khattab) narrated: I went out and proclaimed: Verily none but the believers would enter Paradise.