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. (Using translation from Abū Dāʾūd 2340)
جَاءَ أَعْرَابِيٌّ إِلَى النَّبِيِّ ﷺ فَقَالَ إِنِّي رَأَيْتُ الْهِلَالَ يَعْنِي هِلَالَ رَمَضَانَ فَقَالَ «أَتَشْهَدُ أَنْ لَا إِلَهَ إِلَّا اللَّهُ؟» قَالَ نَعَمْ قَالَ «أَتَشْهَدُ أَنَّ مُحَمَّدًا رَسُولُ اللَّهِ؟» قَالَ نَعَمْ قَالَ «يَا بِلَالُ أَذِّنْ فِي النَّاسِ أَنْ يَصُومُوا غَدًا» تَابَعَهُ سُفْيَانُ الثَّوْرِيُّ وَحَمَّادُ بْنُ سَلَمَةَ عَنْ سِمَاكِ بْنِ حَرْبٍ