Musnad Abu Ya'la al-Mawsili 6:68

Harun ibn Ma'ruf narrated to us, Ibn Wahb narrated to us, Abu Sakhr informed me that Abu Hazim narrated to him, from Ibn Sa'd, who said: I heard my father say: I heard the Messenger of Allah (ﷺ) say: "Indeed, faith began as something strange and it will return as it began. So glad tidings to the strangers on that day when the people become corrupt." "By the One in Whose hand is the soul of Abu al-Qasim, Islam will certainly retreat between these two mosques just as a snake retreats into its hole."