Shatapatha Brahmana 11.2.6.6

11.2.6.6

atha yo'gnīṣomīyaḥ puroḍāśaḥ ayamevāsya sa uttaro'rdha aindraṃ vā sāṃnāyyamantarāṃsamevāsya sviṣṭakṛdviṣam prāśitram

And that cake which is offered to Agni and Soma (at full moon), or Indra's Sânnâyya (at new moon), is its left flank; the Svishtakrit is that part between its shoulders; and the (Brahman's) fore-portion is the poison.