Whether one must make the intention to fast for each day of Ramadan separately is a controversial issue among jurists. The Shafi`is, Hanafis and a group of Hanbali jurists are of the opinion that one has to make the intention to fast each day of Ramadan in the preceding night before dawn, and if one has not done so, one is to make up for the fast of that day.
The Malikis and another group of Hanbali jurists believe that it is sufficient for one to make the intention to fast in Ramadan only once (in the beginning of Ramadan).
to be on the safe side, one is to make the intention to fast each day of Ramadan separately.
Dr. Ahmad Taha Rayyan,, Professor of Comparative Jurisprudence at Al-Azhar University, states: Al-Bukhari and Muslim reported on the authority of `Umar ibn Al-Khattab (may Allah be pleased with him) that Allah’s Messenger (peace and blessings be upon him) said: “Deeds (their correctness and rewards) depend upon intentions, and every person gets but what he has intended. So whoever has emigrated for Almighty Allah and His Messenger (peace and blessings be upon him), his emigration is for Almighty Allah and His Messenger (peace and blessings be upon him), and he who has emigrated for worldly benefits, or for a woman to marry, his emigration is for what he has emigrated for.”
the Shafi`is, Hanafis and a group of Hanbali jurists cited the above hadith as evidence of one’s having to make the intention to fast for each day of Ramadan separately. They gave as a reasoning for their point of view that fasting each day of Ramadan is a separate deed of its own, and deeds, pursuant the hadith cited above, depend on their doers’ intentions. Hence, according to them, one is to make the intention to fast each day of Ramadan separately in the preceding night (from after sunset until dawn). If one has forgotten to make the intention to fast a day in the preceding night until the dawn has come, one is to continue fasting the day but is to make up for it later.
they cite also as evidence of their view in that regard the hadith reported on the authority of Hafsah, Mother of the Believers (may Allah be pleased with her), that the Prophet (peace and blessings be upon him) said: “He who has not made the intention to fast a day in the preceding night his fast is invalid.” (Reported by An-Nasa’i and Abu Dawud) There is also another hadith reported on the authority of `A’ishah, Mother of the Believers (may Allah be pleased with her), that the Prophet (peace and blessings be upon him) said: “He who has not made the intention to fast a day before the dawn his fast is invalid.” (Reported by Ad-Dara Qutni and Al-Baihaqi)
Imam Malik and Imam Ahmad had the point of view that it is sufficient to make the intention to fast the days of Ramadan only once in the beginning of the month. They also cited as evidence of their view the Prophet’s words: “every person gets but what he has intended.” According to them, one has intended to fast the whole month of Ramadan in its beginning, so, it is sufficient for one to make the intention of fasting only once. Besides, they regard fasting Ramadan as an integral act of worship, and breaking fast on the sunset of its days is a means that strengthens one physically to bear fasting along the month.
there are some eminent scholars like Imam Ishaq ibn Rahawiyah and Abu Al-Wafa ibn `Uqail, who supported that point of view, but they said, however, it is desirable for one to renew one’s intention of fast for each day one fasts of Ramadan. (The words of Dr. Ahmad Taha Rayan end here.)

In addition Dr. Rif`at Fawzi, Professor of Shari`ah at Cairo University , states: 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.