It was narrated that Ibn 'Abbas said; "A Bedouin came to the Prphet and said: “I have sighted the crescent.' He said: “Do you bear witness that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger?' He said: “Yes.' So the Prophet gave the call saying: “Fast””””"
DaifDaifDaifBook 22 · Hadith 23