Musnad Abu Ya'la al-Mawsili 150:247

Nasr ibn Ali narrated to us, Ibn Dawud narrated to us, from Thawr ibn Yazid, from Khalid ibn Ma'dan, from Rabi'ah al-Jurashi, from Aisha: That the Messenger of Allah (peace and blessings of Allah be upon him) used to fast Sha'ban and Ramadan, and he would seek to fast on Mondays and Thursdays.