The Messenger of Allâh (sallallâhu ‘alayhi wa-sallam) said: “Fast due to its sighting (i.e the new moon for the month of Ramaḍân) and break your fast due to its sighting. If it is hidden from you then complete the time (of the month of Sh`abān) as thirty days.” (Bukhāri and Muslim)
Imam Ibn Bāz (may Allâh have mercy upon him) said concerning the above ḥadīth:
The meaning of this speech is: That which is obligatory is that they (the Muslims) fast due to the sighting of the crescent and they break their fast due to the sighting of the crescent. It is not for them to fast based upon the calculations nor due to precautionary measures. It’s a must (that the fast starts) from the sighting (of the new moon) or the completion of the time (for the month of Sh`abān). Continue reading