Is the person who sights the new moon of Ramadaan on his own obliged to fast?
Question

                A man saw the new moon of Ramadaan on his own – is he obliged to fast? If that is the case, is there any evidence?.
Praise be to Allah.If a man saw 
    the new moon of Ramadaan on his own, or he saw the new moon of Shawwaal on 
    his own, and he told the qaadi or the people of the city but they did not 
    accept his testimony, should he fast on his own or should he only fast with 
    the people? There are three scholarly points of view: 
1 – He 
    should act upon his own sighting in both cases, so he should fast from the 
    beginning of the month and break the fast at the end on his own. This is the 
    view of Imam al-Shaafa’i (may Allaah have mercy on him). 
But he 
    should do that secretly so that he does not show openly that he is doing 
    something different than the people and so that this will not lead to them 
    thinking badly of him or regarding him as a fast-breaker when they are 
    fasting. 
2 – He 
    should act upon his own sighting at the beginning of the month and fast on 
    his own, but at the end of the month he should not act upon his own 
    sighting, rather he should break the fast when the people do. 
This is the 
    view of the majority of scholars, including Abu Haneefah, Maalik and Ahmad 
    (may Allaah have mercy on them). 
This view 
    was also favoured by Shaykh Ibn ‘Uthaymeen (may Allaah have mercy on him) 
    who said: This is in order to be on the safe side, so that we may be on the 
    safe side with regard to starting and ending the fast. With regard to 
    fasting, we say to him: fast, and with regard to breaking the fast, we say 
    to him: Do not break the fast, rather you should fast. End quote from 
    al-Sharh al-Mumti’, 6/330. 
3 – He 
    should not act upon his own sighting in either case; rather he should start 
    fasting and stop fasting with the people. 
This was the 
    view of Imam Ahmad in one report, and it was the view favoured by Shaykh 
    al-Islam Ibn Taymiyah, He quoted a great deal of evidence to support it, and 
    he said: The third view is that he should start and end the fast with the 
    people. This is the most correct 9?) view, because the Prophet  (peace 
    and blessings of Allaah be upon him) said: “Your fast is the day when you 
    fast, and your breaking the fast is the day when you break the fast, and 
    your sacrifice is the day when you offer the sacrifice.” Narrated by 
    al-Tirmidhi, who said it is hasan ghareeb. Also narrated by Abu Dawood, and 
    by Ibn Maajah, who mentioned breaking the fast and the sacrifice only. Also 
    narrated by al-Tirmidhi from the hadeeth of ‘Abd-Allaah ibn Ja’far from 
    ‘Uthmaan ibn Muhammad from al-Maqbari from Abu Hurayrah, that the Prophet 
     (peace and blessings of Allaah be upon him) said: “The fast is the day 
    when you fast, the breaking of the fast is the day when you break the fast, 
    and the sacrifice is the day when you offer the sacrifice.” Al-Tirmidhi 
    said: This is a hasan ghareeb hadeeth. And some scholars interpreted this 
    hadeeth as meaning that one should start and end the fast with the community 
    and the majority of the people. End quote from Majmoo’ al-Fataawa, 
    25/114. 
He also 
    quoted as evidence the fact that even if he sighted the new moon of 
    Dhu’l-Hijjah on his own, none of the scholars suggested that he should 
    observe the standing at ‘Arafah on his own. 
He said that 
    the basic principle concerning this matter is that the ruling is connected 
    to the new moon of the month. Allaah says (interpretation of the meaning): 
“They ask 
    you (O Muhammad) about the new moons [ahillah]. Say: These are signs to mark 
    fixed periods of time for mankind and for the pilgrimage”

    [al-Baqarah 2:189]
Hilal (pl. 
    ahillah) in Arabic refers to when something is announced and everyone knows 
    about it. If the hilal shows in the sky but people did not recognize it and 
    announce its sighting, then it is not a hilal. 
Similarly 
    the word shahr (month) is taken from the word shuhrah (fame); if it is not 
    well known among the people then the month has not begun. Many people make a 
    mistake with regard to this matter and think that when the moon appears in 
    the sky, that night is the  first of the month, whether it appears to the 
    people and they announce it or not. But that is not the case, rather it has 
    to appear to the people and they have to announce the beginning of the month 
    when they see it. Hence the Prophet  (peace and blessings of Allaah be 
    upon him) said: “Your fast is the day when you fast, and your breaking the 
    fast is the day when you break the fast, and your sacrifice is the day when 
    you offer the sacrifice” i.e., this day on which you announce the beginning 
    or end of the fast or the sacrifice. If you do not recognize it, then no 
    ruling can be based on that. End quote from Majmoo’ al-Fataawa, 
    25/202. 
This view 
    was expressed in a fatwa issued by Shaykh ‘Abd al-‘Azeez ibn Baaz (may 
    Allaah have mercy on him). 
Majmoo’ 
    Fataawa al-Shaykh, 15/72. 
The hadeeth, 
    “The fast is the day when you fast…” was classed as saheeh by al-Albaani 
    (may Allaah have mercy on him) in Saheeh Sunan al-Tirmidhi, no. 561. 
See the 
    views of the fuqaha’ in al-Mughni, 3/47, 49; al-Majmoo’, 
    6/290; al-Mawsoo’ah al-Fiqhiyyah, 28/18. 
And Allaah knows best.