The Adhan of Bilal should not mislead you nor the whiteness (of the pillar) of dawn, for it is not the whiteness of the true dawn, but that of the false dawn which is vertical like a pillar and you can eat food till the streaks of whiteness spread like it.
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).
حَدَّثَنَا أَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ، حَدَّثَنَا وَكِيعٌ، عَنْ هِشَامٍ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، عَنْ زَيْدِ بْنِ ثَابِتٍ، - رضى الله عنه - قَالَ تَسَحَّرْنَا مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم ثُمَّ قُمْنَا إِلَى الصَّلاَةِ . قُلْتُ كَمْ كَانَ قَدْرُ مَا بَيْنَهُمَا قَالَ خَمْسِينَ آيَةً .
We took meal shortly before dawn along with the Messenger of Allah (ﷺ). We then stood up for prayer. I said: How much span of time was there between the two (acts, i. e. taking of Sahri and observing of prayer)? He said (a span of reciting) fifty verses.