Śatapatha Brāhmaṇa ("of a hundred paths") 4.4.5.6

4.4.5.6

athāha sāma gāyeti | sāma brūhīti vā gāyeti tveva brūyādgāyanti hi sāma tadyatsāma gāyati nedidam bahirdhā yajñācarīraṃ nāṣṭrā rakṣāṃsi hinasanniti sāma hi nāṣṭrāṇāṃ rakṣasāmapahantā

He then says, 'Sing the Sâman!' or 'Speak the Sâman!' but let him rather say 'Sing,' for they do sing the Sâman. The reason why he sings the Sâman is that the evil spirits may not injure that body of his outside the sacrifice, for the Sâman is a repeller of the evil spirits.