It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that he (peace and blessings be upon him) said: "Let not the adhan of Bilal prevent any of you from his sahur, for he only calls to wake up those of you who are sleeping and to send back those of you who are standing (in prayer)."
Jarir says that he did not indicate the dawn by pointing lengthwise, but pointed widthwise, because it is not lengthwise.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8021