The Book Giving Description of the Day of Judgement, Paradise and Hell (Kitab Sifat Al-Qiyamah wa'l Janna wa'n-Nar)
Muslim :: Book 39 : Hadith 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)."