It was narrated that Ibn 'Abbas said:

"A Bedouin come to the Prophet and said; 'I have sighted the crescent tonight.' He said: 'Do you bear witness that there is none worthy of worship except Allah, and that Messenger?' He said: 'Yes.' He said: 'O Bilal, announce to the people that they should fast tomorrow. "'

أَخْبَرَنَا مُوسَى بْنُ عَبْدِ الرَّحْمَنِ، قَالَ حَدَّثَنَا حُسَيْنٌ، عَنْ زَائِدَةَ، عَنْ سِمَاكٍ، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ، قَالَ جَاءَ أَعْرَابِيٌّ إِلَى النَّبِيِّ ﷺ فَقَالَ أَبْصَرْتُ الْهِلاَلَ اللَّيْلَةَ . قَالَ " أَتَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَأَنَّ مُحَمَّدًا عَبْدُهُ وَرَسُولُهُ " . قَالَ نَعَمْ . قَالَ " يَا بِلاَلُ أَذِّنْ فِي النَّاسِ فَلْيَصُومُوا غَدًا " .