Śatapatha Brāhmaṇa ("of a hundred paths") 3.5.3.23

3.5.3.23

atha madhyamaṃ cadirupaspṛśya vācayati | pra tadviṣṇu stavate vīryeṇa mṛgo na bhīmaḥ kucaro giriṣṭhāḥ yasyoruṣu triṣu vikramaṇeṣvadhikṣiyanti bhuvanāni viśvetīdaṃ haivāsyaitacīrṣakapālaṃ yadidamupariṣṭādadhīva hyetatkṣiyantyanyāni śīrṣakapālāni tasmādāhādhikṣiyantīti

He then makes (the sacrificer) say, after touching the middle reed-mat (Vâg. S. V, 20; Rig-veda I, 154, 2), 'Let Vishnu then be praised for his power, terrible like a wild beast prowling about the mountains, on whose three wide strides all beings abide!' Now that (mat-covering) indeed is his (Vishnu, the shed's) upper skull-bone for thereon, as it were, the other skull-bones rest: this is why he says 'they abide on.'