Narrated Abdullah ibn Abbas:

A bedouin came to the Prophet ﷺ and said: I have sighted the moon. Al-Hasan added in his version: that is, of Ramadan. He asked: Do you testify that there is no god but Allah? He replied: Yes. He again asked: Do you testify that Muhammad is the Messenger of Allah? He replied: Yes. and he testified that he had sighted the moon. He said: Bilal, announce to the people that they must fast tomorrow.

حَدَّثَنَا مُحَمَّدُ بْنُ بَكَّارِ بْنِ الرَّيَّانِ، حَدَّثَنَا الْوَلِيدُ، - يَعْنِي ابْنَ أَبِي ثَوْرٍ ح وَحَدَّثَنَا الْحَسَنُ بْنُ عَلِيٍّ، حَدَّثَنَا الْحُسَيْنُ، - يَعْنِي الْجُعْفِيَّ - عَنْ زَائِدَةَ، - الْمَعْنَى - عَنْ سِمَاكٍ، عَنْ عِكْرِمَةَ، عَنِ ابْنِ عَبَّاسٍ، قَالَ جَاءَ أَعْرَابِيٌّ إِلَى النَّبِيِّ ﷺ فَقَالَ إِنِّي رَأَيْتُ الْهِلاَلَ - قَالَ الْحَسَنُ فِي حَدِيثِهِ يَعْنِي رَمَضَانَ - فَقَالَ " أَتَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ " . قَالَ نَعَمْ . قَالَ " أَتَشْهَدُ أَنَّ مُحَمَّدًا رَسُولُ اللَّهِ " . قَالَ نَعَمْ . قَالَ " يَا بِلاَلُ أَذِّنْ فِي النَّاسِ فَلْيَصُومُوا غَدًا " .