Musnad Abu Ya'la al-Mawsili 115:76

Abu Bakr ibn Abi Shaybah narrated to us, Muhammad ibn Bishr al-'Abdi narrated to us, from Sa'id, from Qatadah, from Anas, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Do not let the Adhan of Bilal prevent you from the Suhur, for there is something in his eyesight."