Musannaf Ibn Abi Shayba 21:121

Bakr ibn 'Ubayd narrated to us, 'Isa ibn al-Mukhtar narrated to us, from Muhammad ibn Abi Layla, from Abu Ishaq, from Abu al-Ahwas, from 'Abdullah, that when he would move at night, he would say: "O mankind, there has come to you a proof from your Lord, and We have sent down to you a clear light."