Musnad Ahmad 30:332

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Abu Salih, from Abu Hurairah, or from Abu Sa'id—he (Al-A'mash) was in doubt—who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, Allah has those whom He frees in every day and night, and for every servant among them is a supplication that is answered."