Shatapatha Brahmana 9.1.2.39

9.1.2.39

yajñāyajñiyam pucam | candramā vai yajñāyajñiyaṃ yo hi kaśca yajñaḥ saṃtiṣṭhata etameva tasyāhutīnāṃ raso 'pyeti tadyadetaṃ yajño-yajño 'pyeti tasmāccandramā yajñāyajñiyaṃ candramasamevāsyaitatpucaṃ karotyatho pucamevāsyaitadanasthikamamṛtaṃ karoti

The Yagñâyagñiya (hymn he sings) near the tail;--the Yagñâyagñiya, doubtless, is the moon; for whenever a sacrifice becomes completed, the essence of its oblations goes up to him (the moon); and inasmuch as sacrifice after sacrifice (yagña) goes up to him, the moon is the Yagñâyagñiya: he thus makes the moon his (Agni's) tail, and that tail of his he thus makes boneless and immortal.