The Book of Fasting - Sahih Muslim 1080a

Do not fast till you see the new moon, and do not break fast till you see it; but if the weather is cloudy calculate about it.

Commentary on the Prohibition

This hadith establishes the fundamental principle that the beginning and end of Ramadan are determined by physical sighting of the crescent moon. The command "do not fast" and "do not break fast" carries the force of prohibition, indicating that initiating or concluding the fast based on mere calculation without actual sighting is religiously invalid.

The prohibition serves multiple wisdoms: it preserves the unity of the Muslim community under a single sighting, maintains the simplicity of Islamic worship, and reinforces the concept of tawakkul (reliance on Allah) in matters of worship.

The Exception for Cloudy Conditions

The phrase "but if the weather is cloudy calculate about it" provides the necessary flexibility for circumstances where direct observation is impossible. Scholars interpret "calculate" (faqdurū lahu) as meaning to complete the month of Sha'ban as thirty days, as established in companion narrations.

This exception demonstrates Islam's practical approach to religious obligations, accounting for real-world difficulties while maintaining the primary principle of physical sighting. The calculation here refers to completing the previous month's count rather than astronomical computation.

Scholarly Consensus and Implementation

The majority of classical scholars, including the four Sunni madhahib, agree that moon sighting must be established through reliable visual testimony. Astronomical calculations are only considered when they help determine the impossibility of sighting due to the moon's position.

This hadith forms the basis for the unified practice of Muslim communities beginning and ending Ramadan together, emphasizing the collective nature of this worship and preventing fragmentation in religious observance.