It is narrated from Abdullah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: The month can also be of twenty-nine days. Do not begin fasting until you see the new moon, and do not break the fast until you see (the Shawwal) moon. And if the sky is cloudy, then complete the count to thirty.
Hadith Referenceمسند الإمام الشافعي / كتاب الصيام / 608
Hadith Takhrijاخرجه البخاري، الصوم، باب قول النبي صلى الله عليه وسلم اذا رأيتم الهلال...... الخ (1907)، ومسلم، الصيام، باب وجوب صوم رمضان الرؤية الهلال و الفطر لرؤية الهلال..... الخ (1080)۔
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)۔
Abu Huraira (may Allah be pleased with him) narrates that the Messenger of Allah (peace and blessings be upon him) said: Do not fast one or two days before Ramadan, except for a person who is already in the habit of fasting, then he may fast on those days as well.
Hadith Referenceمسند الإمام الشافعي / كتاب الصيام / 610
Hadith Takhrijاخرجه البخاري، الصوم، باب لا يتقدم رمضان بصوم يوم ولا يومين (1914) ومسلم، الصيام، باب ”لا تقدموا رمضان بصوم يوم ولا يومين“ (1082)۔
It is narrated from Ibn Abbas (may Allah be pleased with him) that he said: I am astonished at those people who begin fasting even before the month (of Ramadan) starts. Whereas the Messenger of Allah (peace and blessings be upon him) said: Do not fast until you see the crescent of Ramadan, and do not break your fast until you see the crescent (of Shawwal).
Hadith Referenceمسند الإمام الشافعي / كتاب الصيام / 611
Hadith Takhrijاخرجه النسائى الصيام ذكر الاختلاف على عمرو بن دينار في حديث ابن عباس فيه (2127) والبيهقي : 207/4 ۔ واحمد : 1/ 221 ، 367 ۔ وصححه ابن الجارود (375)۔