1. It is a condition of the fasting person to have the intention to fast. The intention is not to be uttered on the tongue; rather it is in the heart (your desire to fast).
2. It is Sunnah for the fasting person to eat suhoor, because the Prophet (saws) has said: “Eat suhoor for in suhoor there is blessing.” [al-Bukhaari, 1923; Muslim, 1059]
3. It is Sunnah to delay the suhoor and to hasten in breaking the fast.
Allah said: “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 Sawm (fast) till the nightfall” [Al-Baqarah 2:187]
Al-Bukhaari (1919) and Muslim (1092) narrated from ‘Aa’ishah (ra) that Bilaal used to give the adhaan at night, and the Messenger of Allah (saws) said: ‘Eat and drink until Ibn Umm Maktoom gives the adhaan, for he does not give the adhaan until the dawn comes.”
With regard to what appears on timetables as 'imsaak' – whereing people stop eating roughly 10-15 mins before Fajr Adhaan, the scholars have said that there is no basis for such a thing and goes against the Sunnah.
Ibn Hazm narrated that ‘Umar ibn al-Khattaab used to say: “If two men are uncertain about dawn let them eat until they are certain.”