Śatapatha Brāhmaṇa ("of a hundred paths") 3.5.3.24

3.5.3.24

atha rarāṭyāmupaspṛśya vācayati | viṣṇo rarāṭamasīti lalāṭaṃ haivāsyaitadathocrāyyā upaspṛśya vācayati viṣṇoḥ śnaptre stha iti srakve haivāsyaite atha yadidam paścāccadirbhavatīdaṃ haivāsyaitacīrṣakapālaṃ yadidam paścāt

Thereupon he makes him say, after touching the front-band (Vâg. S. V, 2), 'Thou art Vishnu's fillet;' for it indeed is his fillet. He then makes him say, after touching the two upright hurdles, 'Ye are the corners of Vishnu's mouth;' for they indeed are the corners of his mouth. Then that mat which is behind there, that indeed is that skull-bone of his here behind (viz. the occiput).