Praise be to Allaah.
What is obligatory is to fast when the sighting of the moon is confirmed, even if it is seen by only one trustworthy Muslim, as the Prophet (peace and blessings of Allaah be upon him) issued orders to fast when a Bedouin testified that he had seen the new moon. Interpreting the hadeeth “Fast when you see it and stop fasting when you see it” as meaning that no individual should fast until he sees the new moon for himself is incorrect, because the hadeeth is telling everyone to fast when the sighting is confirmed, even if only one trustworthy Muslim sees the new moon.(Fataawa al-Lajnah al-Daa’imah, 10/94).
Further evidence that the sighting of the new moon by one trustworthy, reliable Muslim is sufficient for fasting to be obligatory on all the people is the hadeeth of Ibn ‘Umar (may Allaah be pleased with him), who said: “The people went out to sight the new moon and I told the Messenger of Allaah (peace and blessings of Allaah be upon him) that I had seen it, so he fasted and told the people to fast.”(Reported by Abu Dawood in his Sunan, Kitaab al-Sawm, Baab fi Shahaadat al-Waahid ‘ala ru’yat hilaali Ramadaan).
Some of those who follow bid’ah delay fasting until after all the other Muslims have started to fast, because of their misguided belief that fasting is not obligatory on a person until he sees the new moon for himself. The ahaadeeth refute this, and we would ask them moreover: what are those who are blind or who have poor eyesight supposed to do? It is as Allaah says (interpretation of the meaning): “…Verily, it is not the eyes that grow blind, but it is the hearts which are in the breasts that grow blind.” [al-Hajj 22:46].
And Allaah is the One Who guides to the Straight Path.
Sheikh Muhammed Salih Al-Munajjid