Musnad Ahmad 937:47

Sufyan narrated to us, [he said] two narrated to us, from Musa ibn Talhah: Muhammad ibn Abd al-Rahman and Hakim ibn Jubayr, from Ibn al-Hutakiyyah, from Abu Dharr, that he said: A man said to the Prophet, peace and blessings of Allah be upon him, [and he mentioned something], so he commanded him to fast the thirteenth, the fourteenth, and the fifteenth.