Cite the scanned version of the original dicionary like this:
Bosworth, Joseph. "sang." An Anglo-Saxon Dictionary: Based on the Manuscript Collections of the Late Joseph Bosworth. Ed. Thomas Northcote Toller. Oxford: Clarendon Press, 1898. 816.
.I a.add :-- Þæt hé sceolde þone sang lǽran tó twelf mónþum; quatenus cursum canendi annuum edoceret, Bd. 4, 18; Sch. 437, 4. II.add :-- Mid sange gesettendlices rynes and sealteres cum decantatione canonici cursus et psalterii, Angl. xiii. 390, 362. IV.add: (1) the service on a saint's day: cf. mæsse-sang; II :-- On ðone xviiii-an dæg biþ þæs martyres tíd Scĩ Magni ðæs sang (cf. mæssesang, 4) biþ geméted on þám yldran mæssebócum, Shrn. 119, 12.