Reference: Fataawa Ramadhaan – Volume 1, Page 57, Fatwa No.22
Fiqh al-‘Ibaadaat – Page 172
Question: What is the ruling regarding the person who sees the crescent [of the beginning of Ramadhaan] by himself, and [then] did not fast with the people (everyone)?
Response: Whoever sees the crescent by himself, then he is obligated to report it to the Sharee’ah court and testify to this.
The beginning of the month of Ramadhaan is confirmed by the testimony of one person if the judge is satisfied with it and adjudicates in accordance with his testimony. If, however, his testimony is rejected, then some scholars have said that it is obligatory upon him to fast himself since he was certain he saw the crescent. The Prophet (sal-Allaahu ‘alayhi wa sallam) said:
«Fast when you see it (the crescent)…»,
…and this person had seen it.
But some [other] scholars have said that he is not obligated to fast, since the fast is the day the people fast, and breaking the fast is the day the people break the fast (i.e all of them together as united people). And his agreement with the group (everyone) (i.e. fasting with them) is better than his singling himself out and away from them.
Further, other scholars have explained that he should fast in secret, since he did actually see the crescent; And this is so as not to make apparent his differing with the group (everyone).