Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said that a Bedouin came to the Prophet (peace and blessings be upon him) on the night of the crescent of Ramadan and said: O Messenger of Allah, I have seen the crescent. He (peace and blessings be upon him) said: "Do you bear witness that there is no god but Allah and that I am the Messenger of Allah?" He said: Yes. He (peace and blessings be upon him) said: "Then fast."
It was likewise narrated from Abu Asim from al-Thawri as connected (mawsul), while others narrated it from al-Thawri as mursal.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7975