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 upon its appearing and break the fast upon its appearing. If clouds or the like come between you and it, then complete the count, and do not go out to meet the month (that is, do not welcome Ramadan by fasting a day of Sha'ban)."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7947