Cite the scanned version of the original dicionary like this:
Toller, T. Northcote, and Joseph Bosworth. "bæc." An Anglo-Saxon Dictionary: Based on the Manuscript Collections of the Late Joseph Bosworth : Supplement. Oxford: Clarendon Press, 1921. 757.
;g. bæces; pl. nom. acc. bacu, bæc; g. baca; d. bacum; n. A BACK; dorsum, tergum [dorsum is opposed to venter, especially in animals; and tergum to frons, v. hricg] :-- Mínra feónda bæc ðú onwendest to me inimicorum meorum dedisti mihi dorsum, Ps. Th. 17, 38. Fýnd míne ðú sealdest me on bæc vel hricc inimicos meos dedisti mihi dorsum, Ps. Spl. 17, 42; myn enemys thou ȝeue to me bac, Wyc. 17, 41. Ðá wendon hí me heora bæc to then turned they their backs to me, Bt. 2; Fox 4, 13.