Musnad Ahmad 1153:495

Hashim ibn al-Qasim narrated to us, he said: al-Ashja‘i narrated to us, from Sufyan, from Thawr, from Khalid ibn Ma‘dan, from ‘Aishah, that she was asked about the fasting of the Messenger of Allah, may Allah bless him and grant him peace. She said: "He used to fast Sha‘ban, and he would seek out Monday and Thursday."