From Sahih Muslim (Hadith 2545):
Samura b. Jundub reported Allah's Messenger (ﷺ) as saying: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
Reference: Muslim 2545