It wasnarrated that Ibn 'Abbas said:
"A Bedouin came to the Prophet(ﷺ) and said: 'I have seen the new crescent tonight.' He said:'Doyou bear witness that none has the right to be worshipped butAllahand that Muhammad is the Messenger of Allah (ﷺ)?' He said:'Yes.' Hesaid: 'Get up, O Bilal, and announce to the peoplethat they shouldfast tomorrow.'" (FR)
English reference (FR): Vol. 1, Book 7, Hadith 1652 (FR)
Arabic reference (FR): Book 7, Hadith 1721 (FR)