Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) said that the Messenger of Allah (peace and blessings be upon him) said: "Fast Ramadan upon the appearing of it (the crescent) and break the fast upon its appearing. If clouds come between you and it, then complete thirty days of Sha'ban, and do not welcome Ramadan by fasting a day in Sha'ban."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7949