Hadith
حَدَّثَنَا حُمَيْدُ بْنُ مَسْعَدَةَ، حَدَّثَنَا عَبْدُ الْوَهَّابِ، حَدَّثَنِي أَيُّوبُ، قَالَ كَتَبَ عُمَرُ بْنُ عَبْدِ الْعَزِيزِ إِلَى أَهْلِ الْبَصْرَةِ بَلَغَنَا عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم . نَحْوَ حَدِيثِ ابْنِ عُمَرَ عَنِ النَّبِيِّ صلى الله عليه وسلم زَادَ وَإِنَّ أَحْسَنَ مَا يُقَدَّرُ لَهُ إِذَا رَأَيْنَا هِلاَلَ شَعْبَانَ لِكَذَا وَكَذَا فَالصَّوْمُ إِنْ شَاءَ اللَّهُ لِكَذَا وَكَذَا إِلاَّ أَنْ تَرَوُا الْهِلاَلَ قَبْلَ ذَلِكَ .
Narrated Ayyub :'Umar b. 'Abd al-'Aziz wrote (a letter) to the people of Basrah: It has reached us from the Messenger of Allah (ﷺ), like the tradition narrated by Ibn 'Umar from the Prophet (ﷺ). This version adds: The best calculation is that when we sight the moon of Sha'ban on such-and-such date, fasting will being on such-and-such dates, Allah willing, except they they sight the moon before that (date)
Sahih · Al-Albani
Sunan Abi Dawud 2321 ·Book 14, Hadith 9·Also available on sunnah.com·