Matsyapurana 138.40

138.40

ekaṃ tu ṛgvedaturaṃgamasya pṛṣṭhe padaṃ nyasya vṛṣasya caikam
tasthau bhavaḥ sodyatabāṇacāpaḥ purasya tatsaṅgasamīkṣamāṇaḥ //

Śeṣa Giriṣka and Brahmâ began to pierce the limbs of the enemies from the air and thundered loudly Śiva then with His eyes fixed on Tripura rested His one foot on the Ṛigveda personified as a horse and the other one on His Nandi He drew His bow with arrows By the weight of the feet of Śiva both the horse and the bull became oppressed and respectively the breasts and the teeth of the horse and the Nandî bull fell to the ground It is since then that the teeth and the breasts of the bull and the horse are not seen and fixed in a way as to make them invisible—39 42