Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) mentioned the crescent moon and said: "When you see it, fast, and when you see it, break the fast. If it is clouded over for you, then complete thirty days."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8182
Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "Fast upon sighting it and break the fast upon sighting it. If it is clouded over for you, then fast thirty days." [Sahih. See the previous one.]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8183
Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says that the Messenger of Allah (peace and blessings be upon him) commanded us that "we fast upon sighting the crescent and break the fast upon sighting it, and if it is clouded over for us, that we complete thirty days."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8184