The Lajnah ad-Da’imah was asked the following question:
I would like Your Eminence to clarify the meaning of Imsak. Is it only an act of Sunnah (whatever is reported from the Prophet), or are all Muslims required to abide by it? Is it true that the Messenger of Allah (peace be upon him) specified the time of Imsak as being equal to the recitation of fifty Ayahs (Qur’anic verses) before Fajr (Dawn) Prayer? Scholars estimated it to be half an hour. What is the ruling on whoever eats or drinks while the Adhan (call to Prayer) for Fajr is being announced? Does Sawm (Fast) start upon hearing the Adhan for Fajr Prayer or half an hour before or after the Adhan is announced?
Imsak means abstention from food, drink, and all that invalidates Sawm from the beginning of the appointed time of Sawm. Allah (Glorified and Exalted be He) clarifies in the Glorious Qur’an the starting time of Sawm, which is the break of dawn, saying:
…and eat and drink until the white thread (light) of dawn appears to you distinct from the black thread (darkness of night), then complete your Saum (fast) till the nightfall.
The Adhan for Fajr Prayer is a proclamation of the break of dawn. Therefore, you have to stop eating and drinking when you hear it.
May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.
AbdulFattaah Bin Uthman