Shatapatha Brahmana 8.1.3.3

8.1.3.3

tadāhuḥ | yadgrahāya gṛhītāya stuvate 'tha śaṃsatyatha kasmātpurastādgrahāṇāmṛcaśca sāmāni copadadhātīti saṃsthā vai karmaṇo 'nvīkṣitavya 'rcā vai pratipadā graho gṛhyata ṛci sāma gīyate tadasyaitadyatpurastādgrahāṇāmṛcaśca sāmāni copadadhātyatha yadupariṣṭādgrahāṇāṃ stutaśastre bhavatastadvasyaitadyadupariṣṭādgrahāṇāṃ stomāṃśca pṛṣṭhāni copadadhāti

As to this they say, 'As they chant and recite for the cup when drawn, wherefore, then, does he put in verses and hymn-tunes before (the drawing of) the cups?'--Doubtless, the completion of the sacrificial work has to be kept in view;--now with the opening hymn-verse the cup is drawn; and on the verse (rik) the tune (sâman) is sung: this means that he thereby puts in for him (Agni) both the verses and hymn-tunes before (the drawing of) the cups. And when after (the drawing of) the cups there are the chanting (of the Stotra) and the recitation (of the Sastra): this means that thereby he puts in for him both the stomas (hymn-forms) and the prishtha (sâmans) after (the drawing of) the cups.