Shatapatha Brahmana 14.3.1.10

14.3.1.10

athāha sāma gāyeti sāma brūhīti vā gāyeti tveva brūyādgāyanti hi sāma tadyatsāma gāyati nedimānbahirdhā yajñācarīrānnāṣṭrā rakṣāṃsi hinasanniti sāma hi nāṣṭrāṇāṃ rakṣasāmapahantā

He then says (to the Prastotri), 'Sing the Sâman!' or 'Recite the Sâman!' but let him rather say, 'Sing the Sâman!' for they indeed sing the Sâman. When he sings the Sâman it is in order that the fiends, the Rakshas, should not injure these outside the sacrifice, the body; for the Sâman is a repeller of the fiends, the Rakshas.