Samura b. Jundub (Allah be pleased with him) reported Allah's Messenger (ﷺ) as saying:

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).

وَحَدَّثَنِي أَبُو الرَّبِيعِ الزَّهْرَانِيُّ، حَدَّثَنَا حَمَّادٌ، - يَعْنِي ابْنَ زَيْدٍ - حَدَّثَنَا عَبْدُ اللَّهِ، بْنُ سَوَادَةَ الْقُشَيْرِيُّ عَنْ أَبِيهِ، عَنْ سَمُرَةَ بْنِ جُنْدَبٍ، - رضى الله عنه - قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏

‏ لاَ يَغُرَّنَّكُمْ مِنْ سَحُورِكُمْ أَذَانُ بِلاَلٍ وَلاَ بَيَاضُ الأُفُقِ الْمُسْتَطِيلُ هَكَذَا حَتَّى يَسْتَطِيرَ هَكَذَا ‏

‏ ‏.‏ وَحَكَاهُ حَمَّادٌ بِيَدَيْهِ قَالَ يَعْنِي مُعْتَرِضًا ‏.‏