Sahih Muslim Book 001, Hadith Number 0428.
Chapter : This Ummah (Ummah of Islam) would constitute half of the inhabitants of Paradise.
'Abdullah (b. Mas'ud) reported: We, about forty men, were with the Messenger of Allah (may peace be upon him) in a camp when he said: Aren't you pleased that they should constitute one-fourth of the inhabitants of Paradise? He (the narrator) said: Yes. He (the Holy Prophet) again said: Aren't you pleased that you should constitute one-third of the inhabitants of Paradise? They said: Yes. Upon this he again said: By Him in Whose Hand is my life, I hope that you would constitute one-half of the inhabitants of Paradise and the reason is that no one would be admitted into Paradise but a believer and you are no more among the polytheists than as a white hair on the skin of a black ox or a black hair on the skin of a red ox.