Shaykh al-Islam’s view that hilaal is a noun referring to the day when the people decide the beginning of the month and that shahr refers to that which becomes well-known among them, is based on a considerable amount of evidence, as follows:
That Allah connected shar’i rulings to that which is called hilaal and shahr, such as the beginning and end of the fast, and the sacrifice. Allah says (interpretation of the meaning): “They ask you (O Muhammad صلى الله عليه وسلم) about the new moons. Say: These are signs to mark fixed periods of time for mankind and for the pilgrimage” [al-Baqarah 2:183]. Here Allah stated that the new moons (ahillah, pl. of hilaal) are signs to mark fixed periods of time for mankind and for the pilgrimage. And Allah says (interpretation of the meaning): “Observing As-Sawm (the fasting) is prescribed for you… The month of Ramadan in which was revealed the Qur’aan, a guidance for mankind” [al-Baqarah 2:183, 185]. The word hilaal (new moon) is taken from istihlaal, which means raising the voice; the word shahr (month) is taken from ishtihaar, which means that which becomes well-known. What people do not raise their voices about and that does not become well-known among them cannot be hilaal or shahr.
Analogy with the new moon of Dhu’l-Hijjah. Shaykh al-Islam said: I have never known of any one who said that the one who sees it should stand by himself [in ‘Arafah] in exclusion of the other pilgrims, or offer his sacrifice or stone Jamrat al-‘Aqabah or exit ihram in exclusion of the other pilgrims. What difference is there between the hilaal of Ramadaan and the hilaal of Dhu’l-Hijjah? How come he may act on the basis of his own sighting and differ from the main body of Muslims in Ramadan, and not act on the basis of his own sighting with regard to the hilaal of Dhu’l-Hijjah?
The hadeeth: “The fast is the day you fast, the breaking of the fast is the day you break the fast, and sacrifice is the day you offer your sacrifice.” Narrated by al-Tirmidhi (697); classed as saheeh by al-Albaani in Silsilat al-Ahaadeeth al-Saheehah, 244. What the hadeeth means is that the fast, the breaking of the fast and the sacrifice should be done with the main body of the Muslims, and no one should act on his own with regard to any of that. Hence Imam Ahmad said, according to one report from him: One should fast with the ruler and the main body of the Muslims. The Hand (i.e., support) of Allaah is with the jamaa’ah (main body of Muslims).
This evidence offers strong support for the view favoured by the Shaykh (may Allah have mercy on him).
See: Majmoo’ Fataawa Ibn Taymiyah (25/109-118).
Undoubtedly this issue is one of those that are subject to ijtihaad and the scholars differed concerning it. What is required of the Muslim who is able to examine issues and find out which is more sound on the basis of evidence is to act upon that which he believes to be more sound.
If you do not feel comfortable with the evidence mentioned above, and you think that the opinion of those who say that the one who sights the new moon of Ramadan on his own should fast is more correct, then you have to act upon that which is more correct in your view.
Shaykh Ibn ‘Uthaymeen (may Allah have mercy on him) said: If a person is certain that the month has begun because he has sighted the new moon, but he was not able to inform the sharee’ah court, does he have to fast?
He replied: The scholars differed concerning that. Some of them said that he does not have to fast. That is based on the view that hilaal is a noun referring to the day when the people decide the beginning of the month and that shahr refers to that which becomes well-known among them.
And some of them say that he does have to fast, because the hilaal is that which is seen after the setting of the sun, whether it becomes well known among the people or not.
What appears to me to be the case is if a person sees it and is certain of seeing it, but he is in a remote place where no one else sees it with him, or no one else came out with him to sight it, then he has to fast, because of the general meaning of the words of Allah (interpretation of the meaning): “So whoever of you sights (the crescent on the first night of) the month (of Ramadan i.e. is present at his home), he must observe Sawm (fasts)” [al-Baqarah 2:185], and because the Prophet (blessings and peace of Allah be upon him) said: “ If you see it, then fast.” But if he is in a city and he testifies to that in a court but his testimony is not accepted, then in this case he should fast secretly, so that he does not openly differ from the people. End quote from Majmoo’ Fataawa al-Shaykh Ibn ‘Uthaymeen, 19/74
And Allah knows best.