Agni Purana 269.1

269.1

tripurañjaghnuṣaḥ pūrvaṃ brahmaṇā viṣṇupañjaraṃ /
śaṅkarasya dvijaśreṣtha rakṣaṇāya nirūpitaṃ //

Anciently, when Śaṅkara set out to slay Tripura, the Viṣṇu-armour was prescribed / by Brahmā, O best of the twice-born, for his protection;