Sahih Muslim
The Book Pertaining to the Remembrance of Allah, Supplication, Repentance and Seeking Forgiveness (Kitab Al-Dhikr)
Book Number: 035 / Hadith Number: 6552
Abu Huraira reported that Allah's Messenger (may peace be upon him) used to command us that as we go to our bedo, we should utter the words (as mention- ed above) and he also said (these words):" From the evil of every animal, Thou hast hold upon its forelock (Thou bast full control over it)."