Fiqh-us-Sunnah Volume 3, Zakaat and Fasting, Fiqh 3.113.

Section : Sighting of the Crescent by one Person.

The scholars of fiqh agree that if only one person sees the new moon, he is to fast. ‘Ata differs and says that he is not to fast until someone else also sights the new moon with him. The correct position is that he is to break the fast, as ash-Shaf’i and Abu Thaur have ruled. The Prophet has based the fast and its breaking on the sighting of the moon. One’s own sight is enough for him and there is no need for another person’s sighting.