It was narrated from Abu Dharr that the Prophet said to a man:"You should fast the thirteenth, fourteenth and fifteenth." (Hasan) Abu 'Abdur-Rahman (An-Nasai) said: This is a mistake, it is not a narration of Bayan; perhaps Sufyan said: "It was narrated to us by two (Ithnan)" and the 'Alif was dropped so it became Bayan