Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.131.

Section : Fasting, doubt concerning the time of fajr.

If one is in doubt whether or not the time of fajr has begun or not, he may continue to eat and drink until he is certain that it is fajr. He should not base his action on doubt or suspicion. Allah has made the signs for beginning the daily fast very clear and unambiguous. Allah enjoins (upon the believers) in the Qur’an: “Eat and drink until the white thread of the dawn becomes distinct from the black thread [of the night].”

A man said to Ibn ‘Abbas: “I eat until I suspect that its time has ended so I stop. Ibn ‘Abbas observed: “Continue to eat until you are certain about the time.” Abu Dawud reported that Ahmad ibn Hanbal said: “If you have some doubt about fajr, eat until you are sure dawn has come.” This is the opinion of Ibn ‘Abbas, ‘Ata, al’Auza’i, and Ahmad.

An-Nawawi informs that: “The followers of ash-Shafai agree that one may eat if he is uncertain whether dawn has come or not.”