Musnad Abu Ya'la al-Mawsili 209:92

Abu Khaythama narrated to us, Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from Mansur, from Salim, from Abu Salama, from Umm Salama, who said: "I did not see the Messenger of Allah (peace and blessings of Allah be upon him) fast a full month, except that he used to join Sha'ban with Ramadan."