Proof

Just prior to the Siege of Trenzalore, the Eleventh Doctor presented a severed Dalek eyestalk as "proof of courage and comradeship" aboard what he soon found to be a Dalek ship. (TV: The Time of the Doctor)

Dave Tonbridge had a photo of the wedding of his parents to proove that the Ninth Doctor was their best man. (PROSE: Have You Seen This Man?)