Śatapatha Brāhmaṇa ("of a hundred paths") 4.4.3.15

4.4.3.15

atha mukhānyupaspṛśante | dvayaṃ tadyasmānmukhānyupaspṛśante 'mṛtaṃ vā āpo 'mṛtenaivaitatsaṃspṛśanta etadu caivaitatkarmātmankurvate tasmānmukhānyupaspṛśante

They then touch their faces. There is a twofold reason why they touch their faces;--water, forsooth, is the elixir of immortality: it is with the elixir of immortality that they thus touch themselves. And, moreover, they thus deposit that holy work into their own self: therefore they touch their faces.