By Imam Muhammad Naasir-ud-Deen Al-Albaanee
Abu Hurayrah (raa) related that the Prophet sallallahu alaihi wa sallam said:
“Fast when they fast, end the fast when they end theirs, and sacrifice the day that they sacrifice.” 
Al-Bayhaqee relates by way of Abu Haneefah, who said: ‘Alee ibn al-Aqmar related to me, from Masrooq, who said: I entered upon ‘Aaishah on the day of ‘Arafah, so she said: “Serve Masrooq with some gruel, and make it more sweet.” Masrooq said: Nothing prevented me from fasting this day except that I feared that it may be the day of Sacrifice. So ‘Aa’ishah said to me: “The day of Sacrifice is when the people sacrifice, and the day of ending the fast is when the people end their fast.” This chain of narration is jayyid (good) due to what has preceded.
Understanding This Hadeeth:
Imaam at-Tirmidhee says after quoting the hadeeth: “One of the people of knowledge has explained this hadeeth by saying: Its meaning is to fast and end the fast along with the Jamaa’ah and the majority of people.”
As-San’aanee said in Sublus-Salaam (2/72): “In this (hadeeth) is a proof that being in agreement with the people is accepted in establishing ‘Eed, and that the individual person who believes that it is the day of ‘Eed – because of the sighting of the moon – then it is obligatory upon him to be in agreement with the people, and that the ruling of the people – concerning the Prayer, breaking the fast, and sacrificing – is binding upon the individual.”
Ibn al-Qayyim (rahimahullaah) mentioned this meaning in Tahdheebus-Sunan (3/214), and said: “It is said: In it a refutation of those who say that whosoever knows the positions of the moon due to astronomical calculations, then it is permissible for him to fast and end the fast, even if others do not know. It is also said: That the individual witness who sees the moon, but the qaadee (judge) has not accepted his testimony, then there is no fasting for him, just as there is no fasting for the people.”
Abul-Hasan as-Sindee said in Haashiyah ‘alaa Ibn Maajah, after mentioning the hadeeth of Abu Hurayrah which was related by at-Tirmidhee: “And its apparent meaning is: That there is no room for individual (opinions) to enter into these affairs, nor to act individually in this. Rather, this affair goes back to the Imaam (the Leader of the Muslims) and the Jamaa’ah (united body of Muslims under the Imaam). It is obligatory upon the individuals to follow the Imaam and the Jamaa’ah. From this is that is an individual sights the moon, but the qaadee rejects his witness, then the individual has no right in these matters, but rather he must follow the Jamaa’ah in this.”
And this is the meaning which is evident from the hadeeth, and which is emphasized by the fact that ‘Aa’ishah (raa) used it with Masrooq when he prevented himself from fasting on the day of ‘Arafah, fearing that it could be the day of Sacrifice. So she explained to him that there is no weight given to his individual opinion in this, and that he should follow the Jamaa’ah. So she said to him: “The day of Sacrifice is when the people sacrifice, and the day of ending the fast is when the people end their fast.”
Unity is One of the Goals of the Sharee’ah
And this is what is befitting for the easy-natured and tolerant Sharee’ah (Prescribed Islaamic Law), one of the goals of which is uniting the people together, unifying their ranks and keeping away from them all that would split their comprehensive unity – from the individual opinions. So the Sharee’ah does not give any weight to the individual opinion in matters concerning ‘ibaadah jamaa’iyyah (collective acts of worship), such as Fasting, ‘Eed and Prayer in congregation – even if the opinion is correct, from one angle. Do you not see that the Sahaabah (the Companions) – radiallaahu ‘anhum – used to pray behind each other. So from them were those who held the view that touching a woman, or the flowing of blood from the body invalidates the wudhoo’ (ablution), along with those who did not hold this view. From them were those who would complete the Prayer whilst traveling, whilst others shortened. Yet these, and other such differences, did not prevent them from collectively praying behind a single Imaam and deeming it to be acceptable. And this is because they knew that tafarruq (splitting-up) in the Religion is more evil than having ikhtilaaf (differences) in some opinions. Indeed, the matter with one of them reached the extent that he would not even deem acceptable any opinion which differed with the great Imaam in the major gatherings; such as the gathering at Minaa (during Hajj), to the extent that he would totally abandon acting upon his opinion in that gathering – fleeing from that which could result from this evil, because of acting according to his own opinion.
Thus, Abu Daawood relates (1/307) that ‘Uthmaan (raa) prayed four rak’ahs at Minaa, so ‘Abdullaah ibn Mas’ood criticized him saying: “I prayed two rak’ahs with the Prophet (Sallallaahu ‘alaihi wa Sallam) and two rak’ahs with Abu Bakr, and two rak’ahs with ‘Umar, and two rak’ahs with ‘Uthmaan in the beginning of his rule, then he completed it (i.e. by praying four rak’ahs). After that the ways became divided with you all. So I hope from these four rak’ahs, that two of them would be accepted.” Then Ibn Mas’ood prayed four rak’ahs. So it was said to him: You criticized ‘Uthmaan, yet you prayed four? So he said: “Differing is evil.”
Its chain of narration is Saheeh (authentic), and something similar to this is related in the Musnad (5/155) of Imaam Ahmad, from Abu Dharr (raa).
So those who continue splitting-up with regards to the Prayer, and who refuse to follow the local Imaams in some mosques – especially in the witr Prayer during Ramadaan – using as proof that this is against their madhhab (school of thought), then they should reflect upon the above mentioned hadeeth and athar (narration). Likewise, those who claim knowledge of astronomy and who, due to their opinion, fast and end their fast alone – preceding or lagging behind the majority of Muslims, not seeing any problem in doing so – should also reflect upon the previously quoted proofs. So all of them should consider, and reflect upon the knowledge that has been mentioned. Perhaps they will find for themselves a cure for their ignorance and self-delusion, so that they may then become a unified rank along with their Muslim brothers – for indeed the Hand of Allaah Subhanahu wa Taa’ala is over the Jamaa’ah.
 Silsilatul-Ahaadeethus-Saheehah (1/442-445), the hadeeth authentications have been abridged and edited.
 Saheeh: Related by at-Tirmidhee (2/37). Shaykh al-Albaanee authenticated it in As-Saheehah (no.224)