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) and said: I have seen the crescent of Ramadan. He (peace and blessings be upon him) said: "Do you bear witness that there is no god but Allah?" He said: Yes. He (peace and blessings be upon him) said: "Do you affirm that Muhammad (peace and blessings be upon him) is the Messenger of Allah?" He said: Yes. He (peace and blessings be upon him) said: "O Bilal, announce among the people that they should fast tomorrow."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7973