Ibn ‘Abbas (RAA) narrated, ‘A Bedouin came to the Prophet (ﷺ) and said, ‘I have sighted the crescent (of Ramadan). He then said to him, "Do you testify that there is none worthy of worship but Allah?” The Bedouin said, ‘Yes.’ He asked him again, "Do you testify that Muhammad is the Messenger of Allah (ﷺ)?" The man replied, ‘Yes.’ The Messenger of Allah (ﷺ) there upon said, "O Bilal! Announce to the people to (start) fasting tomorrow." Related by the five Imams. Ibn Khuzaimah and Ibn Hibban graded it as Sahih. An-Nasa'i said that it is most probably Mursal (a Hadith in which the link between the Successor (Tabi’i) and the Prophet (ﷺ) is missing.

وَعَنِ اِبْنِ عَبَّاسٍ رَضِيَ اَللَّهُ عَنْهُمَا أَنَّ أَعْرَابِيًّا جَاءَ إِلَى اَلنَّبِيِّ ‏- صلى الله عليه وسلم ‏-فَقَالَ: { إِنِّي رَأَيْتُ اَلْهِلَالَ, فَقَالَ: " أَتَشْهَدُ أَنْ لَا إِلَهَ إِلَّا اَللَّهُ? " قَالَ: نَعَمْ.‏ قَالَ: " أَتَشْهَدُ أَنَّ مُحَمَّدًا رَسُولُ اَللَّهِ? " قَالَ: نَعَمْ.‏ قَالَ: " فَأَذِّنْ فِي اَلنَّاسِ يَا بِلَالُ أَنْ يَصُومُوا غَدًا" } رَوَاهُ اَلْخَمْسَةُ, وَصَحَّحَهُ اِبْنُ خُزَيْمَةَ, وَابْنُ حِبَّانَ [1]‏ وَرَجَّحَ النَّسَائِيُّ إِرْسَالَهُ [2]‏ .‏