Sahih Bukhari Volume 008, Book 078, Hadith Number 637.
Narated By Abdullah bin Masud : While Allah's Apostle was sitting, reclining his back against a Yemenite leather tent he said to his companions, "Will you be pleased to be one-fourth of the people of Paradise?" They said, 'Yes.' He said "Won't you be pleased to be one-third of the people of Paradise" They said, "Yes." He said, "By Him in Whose Hand Muhammad's soul is, I hope that you will be one-half of the people of Paradise."