Ibn ‘Abbas (RA) told a desert Arab coming to the Prophet (SAW) and saying he had seen the new moon, i.e. the new moon of Ramadan. He asked him whether he testified that there is no god but God and he replied that he did. He then asked if he testified that Muhammad is God’s messenger, and when he replied that he did he said, “Bilal, announce to the people that they must fast tomorrow.” Abu Dawud, Tirmidhi, Nasa’i, Ibn Majah and Darimi transmitted it.
Mishkat al-Masabih 1978, Book 7, Hadith 22