Musnad Ahmad 1153:496

Abdullah ibn Ahmad said: "I found this hadith in my father's book in his own handwriting." Muhammad ibn Humayd Abu Sufyan narrated to us, from Sufyan, from Thawr ibn Yazid, from Khalid ibn Ma'dan, from Aisha: "That the Messenger of Allah, may Allah bless him and grant him peace, used to be particular about fasting in Sha'ban, and fasting on Mondays and Thursdays."