Shatapatha Brahmana 3.5.1.36

3.5.1.36

so 'bhyukṣati | siṃhyasi sapatnasāhī devebhyaḥ śundhasvetyatha sikatābhiranuvikiratyalaṃkāro nveva sikatā bhrājanta iva hi sikatā agnervā etadvaiśvānarasya bhasma yatsikatā agniṃ vā asyāmādhāsyanbhavati tatho haināmagnirna hinasti tasmātsikatābhiranuvikirati so 'nuvikirati siṃhyasi sapatnasāhī devebhyaḥ śumbhasvetyathaināṃ cādayati sā cannaitāṃ rātriṃ vasati

He sprinkles it with, 'Thou art a lioness, overcoming the enemies: get thee pure for the gods!' He then bestrews it with gravel. Now gravel certainly is an ornament, because gravel is rather shining. And that gravel being the ashes of Agni Vaisvânara, he is now about to place Agni thereon, and so Agni does not injure it: this is why he bestrews it with gravel. He bestrews it with, 'Thou art a lioness, overcoming the enemies: array thee for the gods!' He then covers it, and thus covered it remains during that night.