Fasting and other acts of worship are from those acts which must be preceded by intention. 
The Prophet (salallaahu ‘alayhi wa sallam) said:“Verily actions are but by intentions, and for everyone is what he intended”.
And in another narration: “There is no act except with intention”.
So to fast in Ramadan, it is obligatory to make the intention during the night sometime before the arrival of Fajr of the day being intended to fast. A Muslim’s rising up from sleep in the last part of the night and eating the Suhoor is an indication of the presence of the intention. It is not required that a person utter the intention by the tongue by saying: “I intend to fast”. This is an innovation (bid’ah) and is not permissible. The intention in Ramadan is to be done every day (for the next day’s fast) because each day is a separate act of worship requiring its intention. So the intention for the fast is to be made in the heart for every day during the night. If someone makes the intention during the night, then sleeps and does not wake up until after the time of Fajr has entered, then his fast is valid due to the presence of the intention from the night.
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