Sayyiduna Abu Hurayrah (may Allah be pleased with him) narrates that he (peace and blessings be upon him) said: "Fast upon sighting it." Then he narrated the hadith in the same way to the end, and he did not mention that the month is twenty-nine days.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 8207