Sayyiduna Hudhayfah (may Allah be pleased with him) said that the Messenger of Allah (peace and blessings be upon him) said: "Do not precede the month until you see the crescent or complete the count; then fast until you see the crescent or complete the count."
Jarir narrated it from Mansur as connected, mentioning Hudhayfah in it, and he is thiqah (trustworthy) and a hujjah (authority); al-Thawri and a group narrated it from Mansur, from Rib'i, from one of the Companions of the Prophet (peace and blessings be upon him), from the Prophet (peace and blessings be upon him).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7950