Musnad Ahmad 1156:91

'Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from Mansur, from Salim ibn Abi al-Ja'd, from Abu Salamah, from Umm Salamah, who said: "I did not see the Messenger of Allah, may Allah bless him and grant him peace, fast two consecutive months, except that he used to join Sha'ban with Ramadan."