Shatapatha Brahmana 3.8.5.8

3.8.5.8

atha hṛdayaśūlenāvabhṛthaṃ yanti | paśorha vā ālabhyamānasya hṛdayaṃ śukṣamabhyavaiti hṛdayāddhṛdayaśūlamatha yacṛtasya paritṛndanti tadalaṃjuṣaṃ tasmādu paritṛdyaiva śūlākuryāttattriḥpracyute paśau hṛdayam pravṛhyottamam pratyavadadhāti

Thereupon they betake themselves, with the heart-spit, to the purificatory bath. Now, the anguish of the victim, in being slaughtered, concentrates itself into the heart, and from the heart into the heart-spit; and whatever part of cooked (food) is pierced that becomes palatable: therefore let him roast it on the spit after piercing it. Uppermost on the thrice-moved (portions of the) victim he places that heart after pulling it off (the spit).