Musnad Abu Ya'la al-Mawsili 154:6

Suwayd ibn Sa'id narrated to us, Baqiyyah ibn al-Walid narrated to us, from Abu Bakr, he said: Muhammad ibn Yazid narrated to me, from Hanash al-San'ani, from Ibn Abbas, he said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever fasts Wednesday and Thursday, an acquittal from the Fire is written for him."