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.  

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  

Once the people doubted the appearance of the moon of Ramadan, and intended neither to offer the tarawih prayer nor to keep fast. A bedouin came from al-Harrah and testified that he had sighted the moon. He was brought to the Prophet ﷺ. He asked: Do you testify that there is no god but Allah, and that I am the Messenger of Allah? He said: Yes; and he testified that he had sighted the moon. He commanded Bilal who announced to the people to offer the tarawih prayer and to keep fast.  

عَنْ عِكْرِمَةَ أَنَّهُمْ شَكُّوا فِي هِلاَلِ رَمَضَانَ مَرَّةً فَأَرَادُوا أَنْ لاَ يَقُومُوا وَلاَ يَصُومُوا فَجَاءَ أَعْرَابِيٌّ مِنَ الْحَرَّةِ فَشَهِدَ أَنَّهُ رَأَى الْهِلاَلَ فَأُتِيَ بِهِ النَّبِيُّ ﷺ فَقَالَ أَتَشْهَدُ أَنْ لاَ إِلَهَ إِلاَّ اللَّهُ وَأَنِّي رَسُولُ اللَّهِ قَالَ نَعَمْ وَشَهِدَ أَنَّهُ رَأَى الْهِلاَلَ فَأَمَرَ بِلاَلاً فَنَادَى فِي النَّاسِ أَنْ يَقُومُوا وَأَنْ يَصُومُوا  

The people looked for the moon, so I informed the Messenger of Allah ﷺ that I had sighted it. He fasted and commanded the people to fast.  

تَرَاءَى النَّاسُ الْهِلاَلَ فَأَخْبَرْتُ رَسُولَ اللَّهِ ﷺ أَنِّي رَأَيْتُهُ فَصَامَهُ وَأَمَرَ النَّاسَ بِصِيَامِهِ