Musnad Ahmad 1153:571

Haywah ibn Shurayh narrated to us, he said: Baqiyyah narrated to us, he said: Buhayr ibn Sa’d narrated to me, from Khalid ibn Ma’dan, from Jubayr ibn Nufayr, that a man asked ‘Aishah about fasting. She said: "Indeed, the Messenger of Allah, may Allah bless him and grant him peace, used to fast Sha’ban, and he used to seek out the fasting of Thursday and Monday."