The Adhan of Bilal may not mislead you with regard to your food at the commencement of the fast, nor the vertical (streaks) of whiteness in the horizon (for it is an indication of false dawn). You should stop eating (food) till (the whiteness) spreads like it. Hammad narrated it and with the gesture of his band he explained, the horizontal position (of the streaks of light). (Using translation from Muslim 1094c)
قَالَ رَسُولُ اللهِ ﷺ لَا يَغُرَّنَّكُمْ مِنْ سُحُورِكُمْ أَذَانُ بِلَالٍ وَلَا بَيَاضُ الْأُفُقِ الْمُسْتَطِيلُ حَتَّى يَسْتَطِيرَ هَكَذَا وَحَكَاهُ حَمَّادٌ بِيَدِهِ يَعْنِي مُعْتَرِضًا