Matsyapurana 145.61

145.61

atharvaṛgyajuḥsāmnāṃ vedeṣviha pṛthakpṛthak
ṛṣīṇāṃ tapyatāṃ teṣāṃ tapaḥ paramaduścaram //

Now something will be said about the Manvantara, Chāturhotra and the ways of the four varṇās During each Manvantara fresh Śrutis occur, but the Ṛik, Yajuḥ, and Sāma Vedas rules, Devatas, Stotras (hymns), Homas etc., remain the same as in the preceding Manvantara. Vidhistotra and Agnihotra remain as before Dravvastotra Guṇastotra Karmastotra and Kulastotra originate from the Vedas during every Manvantara. From these the Brahmastotra, i e., the four Vedas Ṛik, Yajuḥ, Sāma and Atharvaṇa spring the fourfold mantras (formulæ) as described in the four ways—57-61

The mantras of the preceding Manvantaras flashed in the hearts of the Ṛiṣis who performed very hard tapasyā. Being roused by the feelings of fear, trouble, moha (delusion) grief, discontent, when the Ṛiṣis began to practise tapasyā with great effort and enthusiasm, the mantras came of themselves to them for their deliverance—62-63