Imam Ibn Baz (may Allah have mercy upon him) said commenting on the statement of the Messenger of Allah (sallallaahu alayhi wa sallam):

Fast due to its sighting (i.e the new moon for the month of Ramadan) and break your fast due to its sighting.  If it is hidden from you, then complete the time (of the month of Sha’baan) as 30 days (Bukhari and Muslim.)

The Shaykh said:

The meaning of this speech is that that which is obligatory is that they (the Muslims) fast due to the sighting of the crescent and that 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 is a must (that the fast starts) from the sighting (of the new moon) or the completion of the time (for the month of Sha’baan)…

The Ahaadeeth concerning this are many indicating the obligation of depending  upon the sighting (of the new moon to start the fast for the month of Ramadan).  It is not permissible to depend upon the calculations. Also there is no fasting merely for seeking (to be sure one starts the fast on time) or by way of thinking (that Ramadan 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 Sha’baan).  This is how Allah azza wa jal 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-Ifhaam Fee Sharh Umdatil-Ahkaam pg. 389-390

Translated by Abu Yusuf Khaleefah
16th of Sha’bān, 1437 (05-23-2016)
Masjid Nur Allah, NYC

Original Arabic

قال الإمام ابن باز رحمه الله: الأحاديث في هذا كثيرة تدل على وجوب اعتماد الرؤية ولا يجوز اعتماد الحساب، ولا الصوم بمجرد التحري والظن، بل لابد من الرؤية أو إكمال العدة، هكذا شرع الله عزوجل، وقد أجمع علماء الإسلام على أنه لا يعتمد الصيام بالحساب.

الإفهام في شرح عمدة الأحكام ص. 390