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)۔