Musnad Ahmad 26:597

Isma'il narrated to us, from Sulayman, 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—or he said: the call of Bilal—prevent any one of you from his Suhur, for he calls—or he said: he proclaims—so that your standing one may return, and to awaken your sleeping one. Then it is not [the case that it is time] to say like this—or he said: like this—until he says like this."