Sayyidah Aishah (may Allah be pleased with her) said that the Messenger of Allah (peace and blessings be upon him) used to keep count of the month of Sha'ban to a degree that he did not observe for any other month. Then he would fast upon sighting the crescent of Ramadan. If it was cloudy, he would complete thirty days and then fast.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7939