Taken from here: http://www.masud.co.uk/ISLAM/bmh/BMH-AQ-offa.htm
Sheikh Abdullah Quilliam, writing as Professor H. M. Léon, M.A., D.C.L., F.S.P., etc. (1916)
In the Numismatical department of the British Museum there is preserved a curious and interesting gold coin, over twelve hundred and thirty years old, on which is inscribed in unmistakable Arabic characters the declaration that ‘There is no Deity but Allah, The One, Without Equal, and Muhammad is the Apostle of Allah,’ and the further declaration, engraved around the margin of the coin, ‘Muhammad is the Apostle of Allah, Who sent him (Muhammad) with the doctrine and the true faith to prevail over every other religion.’
This coin was engraved, struck and issued by Offa, King of Mercia, or ‘Middle England’ (an ancient Anglo-Saxon kingdom, which extended on both sides of the River Trent from the North Sea to Wales), from 757 to 796. The name, originally restricted to the district around Tamworth and Lichfield and the Upper Trent valley, refers to a ‘march,’ a moorland, or frontier, which had to be defended against hostile neighbours; in this case such ‘alien enemies’ being the Welsh, the ‘Ancient Britons,’ who for centuries contended with the Anglo-Saxon invaders for supremacy in that region.