Reference: Fataawa Ramadhaan – Volume 1, Page 55, Fatwa No.19;
Tuhfatul-Ikhwaan bi-Ajwibah al-Muhimmah tata’allaq bi-Arkaan al-Islaam – Pages 162-163
Question: How is the beginning and ending of the month of Ramadhaan established? And what is the ruling regarding a person who saw the new moon by himself [whilst no one else saw it] at the beginning of the month or its ending?
Response: The beginning and ending of the month is established when two trustworthy witnesses [see the new moon]. However, [it has been confirmed that] only its beginning can be established with one witness. This is because it has been authentically narrated that the Prophet ﷺ said:
«So if two witnesses testify, then fast or break your fast»
And it has also been authentically narrated that he ﷺ commanded the people to fast according to the testimony of Ibn ‘Umar, [and on another occasion] according to the testimony of a bedouin. And [in both cases] the Prophet ﷺ did not ask for another witness.
And the wisdom in this, and Allaah [alone] has the Complete knowledge [of all affairs], is to be cautious in the starting and ending of the month, as the people of knowledge have mentioned.
And whoever saw the moon by himself, either for the beginning or the ending of the month, but his testimony was not followed by the people, then in this case he fasts with the rest of the people, and breaks his fast with them. He does not act upon his own testimony, according to the strongest of the opinions of the scholars. This is based on the statement of the Prophet ﷺ:
«The fasting is the day that you start fasting, and the breaking of the fast is the day that you break it, and the [‘Eed] al-Adh.haa is the day that you slaughter your animals»
And Allaah [alone] has the Complete knowledge [of all affairs].