Cite the scanned version of the original dicionary like this:
Toller, T. Northcote, and Joseph Bosworth. "mæsse-ǽfen." An Anglo-Saxon Dictionary: Based on the Manuscript Collections of the Late Joseph Bosworth : Supplement. Oxford: Clarendon Press, 1921. 628.
, es; m. The eve of a festival, e.g. Christmas Eve :-- On sc̃e Michaeles mæsseǽfan. Chr. 1014; Erl. 151, 13. Fæstaþ ðæra háligra martyra mæsseǽfenas. Wulfst. 136, 19.