ابْنَ عُمَرَ ـ رضى الله عنهما ـ عَنِ النَّبِيِّ صلى الله عليه وسلم أَنَّهُ قَالَ “ إِنَّا أُمَّةٌ أُمِّيَّةٌ، لاَ نَكْتُبُ وَلاَ نَحْسُبُ الشَّهْرُ هَكَذَا وَهَكَذَا ”. يَعْنِي مَرَّةً تِسْعَةً وَعِشْرِينَ، وَمَرَّةً ثَلاَثِينَ
Ibn `Umar (may Allah be pleased with him) narrated: The Prophet (ﷺ) said, “We are an illiterate nation; we neither write, nor know astronomical calculations/computations. The month is like this and this, i.e. sometimes of 29 days and sometimes of thirty days.” [Bukhari and Muslim]
Hafidh Ibn Hajar (may Allah have mercy upon him) said: “The ruling of fasting continues [to be dependent on the moon sighting] even if it occurs after them [i.e. the Prophet and Companions] one who knows that [i.e. astronomical calculations]. Rather the apparent text and context denies connecting the ruling to astronomical calculations at all…” [Fathul Baari (4/151)]
Faisal bin Abdul Qaadir bin Hassan
Imam as Sanaani mentioned, “A clear response against them….” and he mentioned the response being the narration of Ibn Umar.
Imam Sideeq Hassan Khan said that calculations in figuring out the days and months due to the stages of the moon is an innovation by consensus of the [Muslim] nation.
Lajnah Daimah said:
“Returning to astronomy in affirming the lunar months, for the starting of acts of worship or exiting from it without using eye sight is an innovation which has no good in it nor does it have any evidence for it in the legislation.”
And Ibn Daqeeq Al Eid said:
“That which I say is that the calculations is not permissible to be relied upon in regards to fasting”
[Benefits taken from Itihaaful Anaam of Shaykh Muhammad Hizaam, page 11-12]
Majid Jawed Al-Afghanee