Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa. – Fatwa No. 2031
Fataawa Ramadhaan – Volume 1, Page 56, Fatwa No.20
Question: What is the method by which the beginning of the lunar month is established?
Response: Authentic ahaadeeth on the authority of the Prophet (sal-Allaahu ‘alayhi wa sallam) indicate that whenever one trustworthy person sees the new moon after the thirtieth night of Sha’baan (i.e. the evening following the 29th day), or a number of trustworthy people see it on the thirtieth night of Ramadhaan (i.e. the evening following the 29th day), then this sighting is assumed.
And by this, the beginning of the month is known without the need to take into account the length of time the new moon is visible after the setting of the sun; whether it is twenty minutes or more or less, since there is no authentic ahaadeeth which indicate a specific time limit for the new moon to disappear after the sunset.
And the Council of Senior Scholars in the Kingdom [of Saudi Arabia] agreed to what we have mentioned.
And with Allaah lies all the success, and may Allaah send prayers and salutations upon our Prophet Muhammad (sal-Allaahu ‘alayhi wa sallam) and his family and his companions.