Musnad Ahmad 1153:2291

Ya'qub narrated to us, he said: My father narrated to us, from Ibn Ishaq, he said: Muhammad ibn Ibrahim ibn al-Harith al-Taymi narrated to me, from Abu Salamah ibn 'Abd al-Rahman, from 'Aishah, she said: "The Messenger of Allah, peace and blessings of Allah be upon him, did not fast in any month more than he used to fast in Sha'ban; for he used to fast it, or most of it."