Shatapatha Brahmana 9.1.2.40

9.1.2.40

atha prajāpaterhṛdayaṃ gāyati | asau vā ādityo hṛdayaṃ ślakṣṇa eṣa ślakṣṇaṃ hṛdayam parimaṇḍala eṣa parimaṇḍalaṃ hṛdayamātmangāyatyātmanhi hṛdayaṃ nikakṣe nikakṣe hi hṛdayaṃ dakṣiṇe nikakṣe 'to hi hṛdayaṃ nedīya ādityamevāsyaitaddhṛdayaṃ karotyatho hṛdayamevāsyaitadanasthikamamṛtaṃ karoti

He then sings the heart of Pragâpati;--the heart assuredly is yonder sun, for he (the sun) is smooth, and the heart is smooth; he is round, and the heart is round. On the body (of the altar) he sings, for the heart is in the body;--at the armpit, for the heart is in (the vicinity of) the armpit;-- at the right armpit, for the heart is nearer thereto: he thus makes the sun his (Agni's) heart, and that heart of his he thus makes boneless and immortal.