It is narrated from Sayyiduna Ibn Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Let not the adhan of Bilal (may Allah be pleased with him) - or he said: the call of Bilal (may Allah be pleased with him) - prevent any one of you from his sahur, for he gives the adhan - or he said: he calls out - so that the one standing (in prayer) may return and the one sleeping may be woken." Then he said: "It is not that he says like this, or like this, until he says like this (i.e., he described the manner)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1789