Brewer's: Dunmow Flitch
The oath administered was in the doggerel subjoined:
You shall swear, by the custom of our confession That you never made any nuptial transgression Since you were married man and wife, By household brawls or contentious strife: Or, since the parish clerk said `Amen.' Wished yourselves unmarried again; Or, in a twelvemonth and a day, Repented not in thought any way. If to these terms, without all fear, Of your own accord you will freely swear, A gammon of bacon you shall receive, And bear it hence with our good leave. For this is our custom at Dunmow well known— The sport is ours, but the bacon your own.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894