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).

The aḥādīth concerning this are many indicating the obligation of depending  upon the sighting (of the new moon to start the fast for the month of Ramaḍân). It is not permissible to depend upon the calculations. Also there’s no fasting merely for seeking (to be sure one starts the fast on time) or by way of thinking (that Ramaḍân has begun). Rather it is a must that there is the sighting  (of the new moon) or the completion of the time (for the month of Sh`abān). This is how Allah (‘azza wa-jall) has legislated  (the matter to be). Indeed the scholars of Islam have a consensus that the (start) of the fast does not depend upon the calculations (of when the new month begins).

Source: Al-Ifhām fī Sharḥ `Umdatil-Aḥkām, pg. 389 – 390

Translated by Abu Yusuf Khaleefah
16th of Sh`abān, 1437 (05-23-2016)
Masjid Nur Allah, Queens, NYC