It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her) that the Prophet (peace and blessings be upon him) never fasted a complete month, except that he (peace and blessings be upon him) used to fast Sha'ban and join it to Ramadan.
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19831