Brihadaranyaka Upanishad 2.5.17

2.5.17

idaṃ vai tanmadhu dadhyaṅṅātharvaṇo'śvibhyāmuvāca tadetadṛṣiḥpaśyannavocadātharvaṇāyāśvinā dadhīce'śvyam̐ śiraḥ pratyairayatam | sa vāṃ madhu pravocadṛtāyantvāṣṭraṃ yaddasrāvapi kakṣyaṃ vāmiti

This is that meditation on things mutually helpful which Dadhyach, versed in the Atharva-Veda, taught the Ashwins. Perceiving this the Rishi said, 'O Ashwins, you set a horse's head on (the shoulders of) Dadhyach, versed in the Atharva-Veda. O terrible ones, to keep his word he taught you the (ritualistic) meditation on things mutually helpful connected with the sun, as also the secret (spiritual) meditation on them.'