An Arabi came to the Prophet ﷺ and said, "I have sighted the crescent moon." Hasan added in his version, meaning the crescent of Ramadan.The Prophet ﷺ asked, "Do you bear witness that there is no god but Allah?"He replied, "Yes."The Prophet ﷺ asked, "Do you bear witness that Muhammad is the Messenger of Allah?"He replied, "Yes."The Prophet ﷺ said, "Bilal, announce to the people that they should fast tomorrow."
حدثنا محمد بن بكار بن الريان حدثنا الوليد يعني ابن أبي ثور ح و حدثنا الحسن بن علي حدثنا الحسين يعني الجعفي عن زائدة المعنى عن سماك عن عكرمة
جاء أعرابي إلى النبي ﷺ فقال إني رأيت الهلال قال الحسن في حديثه يعني رمضان فقال أتشهد أن لا إله إلا الله قال نعم قال أتشهد أن محمدا رسول الله قال نعم قال يا بلال أذن في الناس فليصوموا غدا
In its citation, [its chain of transmission is daif]. Tirmidhi recorded it, in the Book of Fasting, chapter on what has been said regarding fasting based on a single testimony, hadith 691, from the hadith of Walid bin Abu Thawr. Nasai recorded it, hadith 2115, from the hadith of Husayn Jufi. Ibn Majah recorded it, hadith 1652, from the hadith of Zaida, all through the same chain.The chain of Simak from Ikrima is a daif chain, as has been mentioned repeatedly. See hadith 2238.