It was narrated that Ibn ‘Abbas said:
“A Bedouin came to the Prophet ﷺ and said: ‘I have seen the new crescent tonight.’ He said: ‘Do you bear witness that none has the right to be worshipped but Allah and that Muhammad is the Messenger of Allah ﷺ?’ He said: ‘Yes.’ He said: ‘Get up, O Bilal, and announce to the people that they should fast tomorrow.’”

حَدَّثَنَا عَمْرُو بْنُ عَبْدِ اللَّهِ الأَوْدِيُّ، وَمُحَمَّدُ بْنُ إِسْمَاعِيلَ، قَالاَ حَدَّثَنَا أَبُو أُسَامَةَ، حَدَّثَنَا زَائِدَةُ بْنُ قُدَامَةَ، حَدَّثَنَا سِمَاكُ بْنُ حَرْبٍ، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ، قَالَ جَاءَ أَعْرَابِيٌّ إِلَى النَّبِيِّ ـ ﷺ ـ فَقَالَ أَبْصَرْتُ الْهِلاَلَ اللَّيْلَةَ . فَقَالَ " أَتَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَأَنَّ مُحَمَّدًا رَسُولُ اللَّهِ " . قَالَ نَعَمْ . قَالَ " قُمْ يَا بِلاَلُ فَأَذِّنْ فِي النَّاسِ أَنْ يَصُومُوا غَدًا " .