Shatapatha Brahmana 11.5.7.10

11.5.7.10

yanti vā āpaḥ etyāditya eti candramā yanti nakṣatrāṇi yathā ha vā etā devatā neyurna kuryurevaṃ haiva tadaharbrāhmaṇo bhavati yadahaḥ svādhyāyaṃ nādhīte tasmātsvādhyāyo'dhyetavyastasmādapyṛcaṃ vā yajurvā sāma vā gāthāṃ vā kuṃvyāṃ vābhivyāharedvratasyāvyavacedāya

Moving, indeed, are the waters, moving is the sun, moving the moon, and moving the stars; and, verily, as if these deities did not move and act, even so will the Brâhmana be on that day on which he does not study his lesson: therefore one's (daily) lesson should be studied. And hence let him at least pronounce either a Rik-verse or a Yagus-formula, or a Sâman-verse, or a Gâthâ, or a Kumbyâ, to ensure continuity of the Vrata.