Śatapatha Brāhmaṇa ("of a hundred paths") 3.8.1.2

3.8.1.2

athāprībhiścaranti | tadyadāprībhiścaranti sarveṇeva vā eṣa manasā sarveṇevātmanā yajñaṃ sambharati saṃ ca jihīrṣati yo dīkṣate tasya riricāna ivātmā bhavati tametābhirāprībhirāpyāyayanti tadyadāpyāyayanti tasmādāpriyo nāma tasmādāprībhiścaranti

They then proceed with the Âprî (verses). The reason why they proceed with the Âprîs is this. With his whole mind, with his whole self, forsooth, he who consecrates himself prepares and endeavours to prepare the sacrifice. His self is, as it were, emptied out; with those Âprîs they fill it again; and because they fill it therewith, therefore they are called Âprî. For this reason they proceed with the Âprîs.