It is narrated from Abu Huraira (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: Do not fast one or two days before the beginning of Ramadan, except for a person who habitually fasts on that day. Begin fasting upon sighting the (Ramadan) crescent, and break the fast upon sighting the (Shawwal) crescent. If it is cloudy, then complete thirty days.
Hadith Referenceمسند الإمام الشافعي / كتاب الصيام / 609
Hadith Takhrijاخرجه الترمذى، الصوم، باب ماجاء لا تقدموا الشهر بصوم (684) ۔ وقال ”حسن صحیح“ ۔ واحمد: 2/438 ۔ والبيهقي : (207/4)۔