Śatapatha Brāhmaṇa ("of a hundred paths") 3.3.4.29

3.3.4.29

tāmabhimṛśati | varuṇasya ṛtasadanyasītyatha kṛṣṇājinamāstṛṇāti varuṇasya ṛtasadanamasītyathainamāsādayati varuṇasya ṛtasadanamāsīdeti sa yadāha varuṇasya ṛtasadanamāsīdeti varuṇyo hyeṣa etarhi bhavati

He (the Adhvaryu) touches it with, 'Thou art the rightful seat (ritasadanî) of Varuna!' He then spreads on it the black deerskin with, 'Thou art the rightful seat (ritasadanam) of Varuna!' and places him (Soma) thereon with, 'Seat thee on the rightful seat of Varuna!' The reason why he says, 'Seat thee on the rightful seat of Varuna,' is that he (Soma) is now of Varuna's nature.