As far as Islamic Shari`ah is concerned, it is sufficient to make intention in one’s heart. In other words, the fasting person is not required to express his intention to fast in explicit words. Everyone who knows that tomorrow is one of the days of Ramadan and wills to fast, this is considered valid intention even if he does not express the intention to fast in explicit words. Also, when one gets up to take sahur (pre-dawn meal) or even intends to get up but was overcome by sleep, this is considered a valid intention to fast.
Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada, states: Intention is an extremely crucial and integral aspect of worship in Islam; this is true of Prayer, fasting, zakah and Hajj; it also applies to all of the acts of devotion and good works that we perform. In the case of any act of worship (`ibadah), the intention means first to cleanse and purify our motivation, or to put it differently, to make sure that we do what we do purely for the sake of Allah and not for any other worldly purpose or objective. Second, it is also important that we formulate the intention in our minds to do the specific act of worship for the sake of Allah. Therefore, while fasting the month of Ramadan, we must resolve in our minds firmly that we are fasting the days of Ramadan as an act of worship. So long as we have done so in our minds, then we have fulfilled the intention requirement. In other words, it is not necessary to state the above intention in words. If anyone does so, we cannot consider his action as sinful, although there is no need for him to do so. It is best that we stick to the practice of the righteous predecessors who were not in the habit of pronouncing the intention in words in such acts of worship.
now as for the question whether one must formulate intention every single day, this is a controversial issue among scholars. One group insists that it must be done each and every day before dawn, while another group states that it is sufficient if one formulates the intention to fast the whole month of Ramadan in the beginning.
there is no need for us to be very rigid on this issue. Everyone who wakes up for sahur (pre-dawn meal) has already made up in his mind to fast. But still the safest way to go about in such matters is to make sure that we formulate the intention each and every day. By doing so we make sure that our acts of worships are not simply mechanical chores but deliberate acts of worship.