Cite the scanned version of the original dicionary like this:
Toller, T. Northcote, and Joseph Bosworth. "ælf-siden." An Anglo-Saxon Dictionary: Based on the Manuscript Collections of the Late Joseph Bosworth : Supplement. Oxford: Clarendon Press, 1921. 15.
, -sidenn, e; f. The influence of elves or of evil spirits, the nightmare; impetus castalidum, diaboli incubus :-- Ðis is se hálga drænc wið ælfsidene and wið eallum feóndes costungum this is the holy drink against elfin influence and all temptations of a fiend, Lacn. 11; Lchdm, iii. 10, 23. Wið ælfsidenne, L. M. 1, 64; Lchdm, ii. 138, 23.