Shatapatha Brahmana 4.3.2.10

4.3.2.10

tadāhuḥ | gāyatrāṇi vai sarvāṇi savanāni gāyatrī hyevaitadupasṛjamānaiditi tasmātsaṃsiddham prātaḥsavane pratigṛṇīyātsaṃsiddhā hi gāyatryāgacatsakṛnmadvanmādhyandine savana ekaṃ hi sākṣaraṃ hitvāgacattenaivaināmetatsamardhayati kṛtsnāṃ karoti

As to this they say, 'Surely all the Soma feasts pertain to Gâyatrî, since Gâyatrî alone went on increasing.' At the morning feast he should therefore respond with a complete (formula), for complete Gâyatrî returned. At the midday feast (he responds with a formula) containing once (the verb) 'to rejoice (mad),' for she (Trishtubh) came back, leaving one syllable behind; and with that same (formula) he then completes her, makes her whole,