Talk b. Ali (may Allah be pleased with him) tells:

The Messenger of Allah stated:

Do not let the fajr al-kazib (the false day break) prevent you from eating or drinking so eat and drink until fajr al-Sadiq, (the true daybreak) till the true fajr appears. (Abu Dawud, Sawm: 17; Tirmidhi, Sawm: 15)

According to the relation of Bukhari and Muslim from Ibn Mas’ud (may Allah be pleased with him), the Messenger of Allah said “It is a brightness seen as horizontal, but not vertical” when he was describing “the true fajr.” (Bukhari, Adhan: 13, Talaq: 24, Khabar al- Wahid: 1; Muslim: Siam: 40)

Fajr means dawn time. It indicates the time Suhur ends and the Morning Prayer can be performed.

The false fajr is a light brightness seen vertically when close to morning in the horizon and which is visible only for a short time. That brightness disappears and after 20-25 minutes, then the true fajr appears. That time, a horizontal brightness among the horizon line appears. That brightness is also called “breaking of the day.”

Thus, the Prophet (pbuh) advices not to esteem the first brightness seen in the horizon.

The time taking place in the daily calendars and known as “imsaak” is the time of the true fajr. Adhan for the Morning Prayer is called at that time. In this way, both the time of imsaak begins, fasting starts and salat al-fajr is performed.