It was narrated that Ibn 'Abbas said: "A Bedouin come to the Prophet (ﷺ) 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. "'
Source: sunnah.com
Other translations
Arabic
أَخْبَرَنَا مُوسَى بْنُ عَبْدِ الرَّحْمَنِ، قَالَ حَدَّثَنَا حُسَيْنٌ، عَنْ زَائِدَةَ، عَنْ سِمَاكٍ، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ، قَالَ جَاءَ أَعْرَابِيٌّ إِلَى النَّبِيِّ صلى الله عليه وسلم 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 " .
sunnah.com