It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her); she said: "The Messenger of Allah (peace and blessings be upon him) never fasted a complete month, except Sha'ban, which he (peace and blessings be upon him) used to join to Ramadan."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19849