Shatapatha Brahmana 11.7.4.3

11.7.4.3

etadvai paśoḥ saṃjñapyamānasya hṛdayaṃ śukṣamavaiti hṛdayācūlaṃ tadye saha hṛdayena paśuṃ śrapayanti punaḥ paśuṃ śuganuviṣpandeta pārśvata evainatkāṣṭhe pratṛdya śrapayet

For on that occasion the anguish of the victim, in being slaughtered, becomes concentrated in the heart, and from the heart (it flows) into the spit. Thus, if they (were to) cook the animal together with the heart, the anguish would again spread all over the animal: let him therefore cook it (the heart) after spitting it from the side on a stick.