Musnad Ahmad 1156:181

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Tawbah al-‘Anbari, from Muhammad ibn Ibrahim, from Abu Salamah, from Umm Salamah, from the Prophet (peace and blessings of Allah be upon him), that: "He would not fast a complete month of the year, knowing it, except for Sha‘ban, which he would join to Ramadan."