Musnad Ahmad 26:107

Yahya narrated to us, from al-Taymi, from Abu Uthman, from Ibn Mas’ud, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Let not the Adhan of Bilal prevent any one of you from his Suhur, for he calls the Adhan — or he said: he calls out — so that your standing one may return, and your sleeping one may awaken. It is not that he says [it is dawn] like this" — and he joined his hand and raised it — "but rather until he says like this" — and Yahya separated his two index fingers. Abu Abd al-Rahman said: "I did not hear this Hadith from anyone."