Sahih Muslim
The Book Giving Description of the Day of Judgement, Paradise and Hell (Kitab Sifat Al-Qiyamah wal Janna wan-Nar)
Book Number: 039 / Hadith Number: 6736
Anas reported this hadlth through another chain of transmitters and the words are:" It would be said to him: You have told a lie; what had been demanded from you was quite easier than this (the belief in the Oneness of Allah)."