Answered by Shaykh Amjad Rasheed
Is it obligatory to renew the intention to fast Ramadan, each and every day, or is it sufficient to intend on the first day to fast the entire month, without renewing it each day?
In our school, and according to the position of most scholars, it is obligatory to renew the intention in Ramadan each day. This is because every day is considered to be a independent act of worship. The proof of this is that if one day of fasting happened to be invalid, it does not invalidate any other fasts, and that one day is the only one you have to make up.
In the Tuhfa [of Ibn Hajar]: It is a condition to make the intention at night for each fast-day, because it is an independent act of worship. Given this, the intention to fast the whole month will not suffice except for the first day.
However in the Maliki school, it is permissible to intend to fast the whole month. This is also a narrated position from Imam Ahmad. Therefore, the fasting person can, out of precaution, make the intention to fast the whole month on the first day, but he also has to renew the intention every day. Then, if he forgets one day to renew it, the intention he made at the beginning of the month will suffice him, according to the Malikis. And he is allowed to follow them in this point, as was pointed out by Shaykh Ibn Hajar, in Fath al-Jawad and other books.
– Amjad Rasheed
[Translated by Sr. Ruqayya Ahdab]