Śatapatha Brāhmaṇa ("of a hundred paths") 1.6.2.6

1.6.2.6

sa na paurṇamāsaṃ haviḥ | nāmāvāsyamagnīṣomīya eva paurṇamāsaṃ haviḥ sāṃnāyyamāmāvāsyaṃ yajña evaiṣa ubhayatrāvakLpto nedyajñādayānīti nveva purastātpaurṇamāsasya kriyata evamvāmāvāsyasyaitannu tadyasmādatra kriyate

That (cake for Agni) does not constitute the (special) sacrificial food (havis) either at the full-moon, or at the new-moon, sacrifice; since the one for Agni and Soma constitutes the havis at the full-moon, and the Sânnâyya at the new-moon sacrifice. That one (for Agni) constitutes rather the regular (or, corresponding) sacrifice on both occasions, and because of its fearing lest it should become detached from the sacrifice, it is offered up at the beginning of both the full-moon and the new-moon sacrifice: this is the reason why it is offered at this particular time.