The prevalent view in the school of Imam Ahmad—many of his later followers explicitly attribute it to him—is that fasting on that thirtieth day is obligatory as a matter of probable evidence and precaution. They base this on the Prophet’s words, “faqdurū lahū” (“calculate for it”), which they construe to mean: “constrict the month of Shaʿbān and reckon it as twenty-nine days.”

This report from Imam Ahmad is one of the positions unique to the Hanbali school, and it is likewise related from several Companions: Abū Hurayrah, Ibn ʿUmar, ʿĀʾishah, and Asmāʾ.

By contrast, the majority of scholars—including the three imams Abū Ḥanīfah, Mālik, and al-Shāfiʿī—maintain that fasting is not obligatory, and that if a person were to fast that day intending it as part of Ramaḍān, the fast would not count.

Shaykh al-Islām Ibn Taymiyyah adopted this majority view, remarking that the numerous, well-known narrations from Ahmad all point in its favour. The author of al-Furūʿ adds: “I have found no explicit statement from Ahmad imposing the fast, nor any order from him to do so; attributing such an opinion to him is therefore unfounded.” The same position was preferred by leading Hanbali authorities such as Abū ’l-Khaṭṭāb and Ibn ʿAqīl.

The evidence for this stance is the ḥadith recorded by the Two Shaykhs (al-Bukhārī and Muslim) from Abū Hurayrah: “Fast when you sight it, and break the fast when you sight it; but if it is obscured from you, then complete thirty days of Shaʿbān.” This—and similar narrations—makes it clear that the phrase “faqdurū lahū” means, “calculate its reckoning by completing Shaʿbān as thirty days.”

Ibn al-Qayyim examined the question thoroughly in his work al-Hady, upheld the opinion of the majority, refuted the contrary views, and showed that no explicit statement is authentically established from any Companion except Ibn ʿUmar, whose personal practice inclined toward caution and stringency. Shaykh al-Islām Ibn Taymiyyah also noted that declaring the fast of the “day of doubt” obligatory has no basis in the words of Ahmad or any of his students, even though some have assumed otherwise. Ahmad’s explicit, textual position is that one may either fast or refrain from fasting on that day—exactly the view held by Abū Ḥanīfah and by many Companions and Successors. All the underlying principles of the sharia confirm that precaution, in itself, is neither obligatory nor prohibited.

The jurists further disagreed over a related issue: if the crescent is sighted in one locale, must all Muslims everywhere begin the fast? The widely transmitted position of Imam Ahmad and his followers is that fasting becomes obligatory upon all Muslims across the globe, for once the start of Ramaḍān is established, its rulings take effect universally. This is one of the distinctive rulings of the Hanbali school and is likewise the opinion of Abū Ḥanīfah.