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