Musnad Abu Ya'la al-Mawsili 152:170

Abu Khaythama narrated to us, Ismail narrated to us, from al-Taymi, from Abu Uthman, from Abdullah, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Let not the Adhan of Bilal—or he said: the call of Bilal—prevent any one of you from his Suhur, for he calls the Adhan—or he said: he calls out—at night, so that your standing one may return and your sleeping one may awaken." And he said: "It is not that you say like this and like this," and he pointed his hand and raised it, "until he says like this," and he spread his fingers.