Sahih Muslim
The Book of Faith (Kitab Al-Iman)
Book Number: 001 / Hadith Number: 271
It is narrated on the authority of Ibn 'Umar ('Abdullah b. 'Umar) that the Messenger of Allah (may peace be upon him) observed: Verily Islam started as something strange and it would again revert (to its old position) of being strange just as it started, and it would recede between the two mosques just as the serpent crawls back into its hole.