Is the intention to fast valid for one who refrained from food and drink for another reason and then intended to fast on the same day?

A presumptive fast is not valid unless the intention is made the night before. As for a supererogatory fast, scholars have differed on it, holding two views:

1. The view of the majority: It is not a condition to make the intention the night before. It is permissible to make the intention during the day before Zawāl, as long as nothing that invalidates the fast has occurred. They cited as evidence the Hadith (definition) of Aisha, "Then I am fasting," and reports from Companions such as Abu al-Dardaa, Abu Talha, Hudhayfa, and Ibn Abbas. 2. The view of the Malikis and some of the Salaf: It is a condition to make the intention the night before for both obligatory and supererogatory fasts alike. They cited as evidence the Hadith of Hafsa, "Whoever does not gather the intention to fast before Fajr, there is no fast for him." They interpreted the Hadith of Aisha by stating that the Prophet, peace and blessings be upon him, had intended to fast the night before but then became weak from it.

The majority believes that the Hadith of Hafsa — assuming its authenticity as being marfu' (attributed to the Prophet) — is specified by the Hadith of Aisha, and that the Maliki interpretation is far-fetched and strained.