Regarding acting according to sighting in affirming the new Moons and not according to Astronomical Calculations

All praise is due to Allâh alone and may blessings and peace be upon him whom there is no Prophet after him (i.e., Muhammad ). To proceed:

Verily, the Council of Islamic Fiqh Academy has reviewed a copy of a letter from the Society of Islamic Preaching in Singapore during its fourth session held at the General Secretariat of the Muslim World League in Makkah Al-Mukarramah. This session took place between 7th and the 17th of Rabi' Al-Akhir, 1401 AH. The letter was dated Shawwal 16th, 1399 AH, correspond- ing to August 8th, 1979 CE. The letter was addressed to the charg d'affaires of the Royal Embassy of Saudi Arabia there in Singapore. The contents of the letter expressed that there was a difference that arose between this Society and the Islamic Council in Singapore concerning the beginning of the month of Ramadhan and its ending in the year 1399 AH or 1979 CE. This was due to the Society holding the view that the beginning of the month of Ramadhan and its ending should be based upon the Islamically sanctioned sighting, as this is in accordance with the generality of the Islamic evidences. At the same time, the Islamic Council in Singapore held the view that the beginning and ending of the aforementioned Ramadhan should be according to astronomical calculations. The reason for that is the Committee's saying: "In reference to countries in the area of Asia, their sky is covered with clouds, especially in Singapore, thus most of the places for sighting the new moon are blocked from sightings. Therefore, this (calculation) is considered from the matters that are excused and necessary. For this reason it is obligatory to determine (the time of the new moon) by way of calculations."

After the members of the Council of Islamic Fiqh Academy undertook a comprehensive study of this issue in the light of the Islamic texts, the Council of Islamic Fiqh Academy decided to support the Society of Islamic Preaching in its view. This is due to the clarity of the Islamic evidences in this matter.

Likewise, the Committee made a decision in reference to this situation where there are places like Singapore and some areas of Asia and other places where their sky is blocked by what prevents sightings. They decided that the Muslims in these areas and similar places may take the sighting of whoever they trust from the Islamic countries that rely upon visual sightings of the new moon instead of any form of calculations. This is acting according to the Prophet's statement:
"Fast according to its sighting and break your fast according to its sighting. And if it is hidden from you by clouds, then complete the number (of days) as 30."

And his statement:
"Do not fast until you see the new moon or you complete the number (of days as 30). And do not break your fast until you see the new moon or you complete the number (of days as 30)."

And whatever Hadiths have been reported with the same meaning as these two.

Source:
Fatawa Islamiyah, Vol. 3 Pages 230-231- 232