Sayyiduna Sahl bin Sa'd (may Allah be pleased with him) said: "the Prophet (peace and blessings be upon him) would fast until it was said that he does not break his fast, and would leave fasting until it was said that he does not fast, and most of his (peace and blessings be upon him) fasting was in Sha'ban."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5784