Merlin/Arthur spanking fic

I remember reading this fic quite a while ago so I'm a little hazy on the details: it was a canon fic written as an entry Arthur wrote for a book on proper treatment of servants describing how he'd discovered that the best way to keep Merlin obedient was sex/spanking.

From what I can remember, the 'plot' was thus: Arthur and Merlin had sex for the first time, then the next morning Arthur decided that Merlin was getting too impudent or something and spanked him as a punishment. Arthur gets turned on by spanking Merlin and they have sex again. This becomes a regular thing and Arthur observes that Merlin gets moody when they miss a day, then recommends that others follow his example.

Thanks. :)