Imsak Time, Between The Preferred Time and The Permissible One
The time for fasting commences at Subah Sadiq (true dawn) and all scholars agreed with that and also it means the cut-off point for consuming sahur is the arrival of dawn (fajr time). This is based on Quran:
It is made lawful for you to go unto your wives on the night of the fast. They are raiment for you and ye are raiment for them. Allah is aware that ye were deceiving yourselves in this respect and He hath turned in mercy toward you and relieved you. So hold intercourse with them and seek that which Allah hath ordained for you, and eat and drink until the white thread becometh distinct to you from the black thread of the dawn. Then strictly observe the fast till nightfall and touch them not, but be at your devotions in the mosques. These are the limits imposed by Allah, so approach them not. Thus Allah expoundeth His revelations to mankind that they may ward off (evil). (Al-Baqarah: 187)
But in Indonesia and others Muslim country (such Turkey) we've heard about Imsak times. What is this?
Imsak means abstaining from food, drink, etc. Imsak time means to stop eat and drink anything. Most of the time people confuse between the preferred time and the permissible one; the preferred time for imsak, according to scholars, is to finish sahur ten minutes before fajr.
Sheikh Ahmad Kutty give an explanation about Imsak time:
There is no doubt that the cut-off point for consuming sahur is the arrival of dawn (i.e., fajr time). However, the preferred time for abstention is 10 or 15 minutes earlier than that.
The basis for establishing the preferred time of abstention (imsak ) is an authentic tradition from the Prophet (peace and blessings be upon him). One of his Companions narrated the story of taking sahur with the Prophet (peace and blessings be upon him). Upon hearing it, someone asked him, "What was the time gap between your sahur and fajr salah?" He replied, "The time required to read fifty verses of the Qur'an." It is estimated that this time can be 10 or 15 minutes and not more.
However, it must be pointed out that the above mentioned hadith does not state that it is not permissible to consume sahur later than that, for it only states that it was the sunnah of the Prophet (peace and blessings be upon him). As for the permissibility of consuming sahur until the arrival of dawn, it has been established by the clear text of the Qur'an: (Eat and drink until the white thread of dawn becomes distinct from the black thread of night) (Al-Baqarah 2:187).
There are two times for stopping eating during sahur (pre-dawn meal), namely a preferred time and a permitted time.
Preferred time to stop eating is ten minutes before Fajr time. So if the Fajr time is 5:00 a.m., one should stop eating around 4:50 a.m. Permitted time for eating suhur, however, extends up to 5:00 a.m., after which time it is considered unlawful. Therefore, if anyone deliberately did so, the fast is considered broken.
References : Islamonline.net, albalagh.net