Sayyiduna Ibn Umar (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said: "The month (sometimes) is also of twenty-nine days, so do not begin fasting until you see the crescent, and do not stop fasting until you see it. If it is cloudy, then complete the count of thirty days."
Hadith Takhrijاس روایت کی سند صحیح اور حدیث متفق علیہ ہے۔ دیکھئے: [بخاري 1914] ، [مسلم 1082] ، [أبوداؤد 2320] ، [أبويعلی 5999] ، [ابن حبان 3586]
Brief Explanation
(In Explanation of Hadith 1727)
In Lam‘at, Mulla Ali Qari has written under this hadith: The majority of the scholars among the Salaf and the Khalaf act upon this hadith. Some people have said that in the aforementioned hadith, by the word «فَاقْدُرُوْالَهُ», what is meant is the calculation of astronomy; this statement is not correct.
Nowadays, the dates that are given in calendars or almanacs, although those who compile them make every effort, yet for legal (shar‘i) matters, merely their opinion and calculation cannot be relied upon. Especially for Ramadan and the two Eids, the sighting of the crescent (ru’yat-e-hilal) or the testimony of two reliable witnesses is necessary (Dawud Raz rahimahullah).