Musnad Ahmad 31:345

Abu Abd al-Rahman narrated to us, he said: Sa'id ibn Abi Ayyub narrated to us, Abdullah ibn al-Walid narrated to us, from Abu Sulayman al-Laythi, from Abu Sa'id al-Khudri, from the Prophet, may Allah bless him and grant him peace, that he said: "The likeness of the believer and the likeness of faith is like a horse on its tether; it wanders, then it returns to its tether. And indeed, the believer becomes heedless, then he returns to faith."