Brihadaranyaka Upanishad 2.5.16

2.5.16

idaṃ vai tanmadhu dadhyaṅṅātharvaṇo'śvibhyāmuvāca tadetadṛṣiḥ paśyannavocat | tadvāṃ narā sanaye dam̐sa ugramāviṣkṛṇomi tanyaturna vṛṣṭiṃ | dadhyaṅha yanmadhvātharvaṇo vāmaśvasya śīrṣṇā prayadīmuvāceti

This is that meditation on things mutually helpful which Dadhyach, versed in the Atharva-Veda, taught the Ashwins. Perceiving this the Rishi (Mantra) said, 'O Ashwins in human form, that terrible deed called Damsa which you did out of greed, I will disclose as a cloud does rain—(how you learnt) the meditation on things mutually helpful which Dadhyach, versed in the Atharva-Veda, taught you through a horse's head.'