Shatapatha Brahmana 11.2.6.5

11.2.6.5

atha ya āgneyaḥ puroḍāśaḥ ayamevāsya sa dakṣiṇo'rdho hṛdayamevāsyopāṃśuyājaḥ sa yattenopāṃśu caranti tasmādidaṃ guheva hṛdayam

And that cake which is offered to Agni is its right flank; and the low-voiced offering is its heart; and inasmuch as they perform this in a low voice, this heart is, as it were, in secret.