Shaykh Bin Baz was asked:
What affirms the entrance [beginning] and exit [end] of the month of Ramadan? And what is the ruling upon the one who sees the new moon by himself for the beginning or ending of the month?
The entrance [beginning] and exit [end] of the month is confirmed by 2 trustworthy witnesses or more, and the entrance of the month is confirmed by 1 witness alone because it is confirmed from the prophet that he said;“…and if 2 witnesses testify then fast and break the fast”
and it is established from the prophet that he commanded the people to fast upon the testification of ibn Umar and by the testification of the bedouin, and he did not request another witness.
And the wisdom in that and Allah knows best, is caution in the religion in the entering and exiting of the month as the people of knowledge have stated. And whoever sees the new moon alone [himself] in the entering or exiting of the month but his testification is not acted upon then in that case he is to fast with the people and break his fast with the people and he is not to act upon his own witnessing of the new moon based upon the more correct opinion of the scholars due to the statement of the prophet;“fasting is the day you all fast, and the opening of the fast is the day you all open it and the day of slaughtering is the day you all slaughter”
Majmoo Fatawa Shaykh Bin Baz volume 15 page 62
Abu Muadh Taqweem Aslam Nelson