Śatapatha Brāhmaṇa ("of a hundred paths") 2.1.2.11

2.1.2.11

phalgunīṣvagnī ādadhīta | etā vā indranakṣatraṃ yatphalgunyo 'pyasya pratināmnyo 'rjuno ha vai nāmendro yadasya guhyaṃ nāmārjunyo vai nāmaitāstā etatparo 'kṣamācakṣate phalgunya iti ko hyetasyārhati guhyaṃ nāma grahītumindro vai yajamānastatsva evaitannakṣatre 'gnī ādhatta indro yajñasya devataiteno hāsyaitatsendramagnyodheyaṃ bhavati pūrvayorādadhīta purastātkraturhaivāsme bhavatyuttarayorādadhīta śvaḥśreyasaṃ haivāsmā uttarāvadbhavati

He may also set up his fires under the Phalgunîs. They, the Phalgunîs, are Indra's asterism, and even correspond to him in name; for indeed Indra is also called Arguna, this being his mystic name; and they (the Phalgunîs) are also called Argunîs. Hence he overtly calls them Phalgunîs, for who dares to use his (the god's) mystic name? Moreover, the sacrificer himself is Indra, so that he in that case sets up his fires under his own asterism. Indra is the deity of the sacrifice; and accordingly his Agnyâdheya is thereby brought into relation with Indra. He may set up the fires under the first (Pûrva-phalgunîs)--whereby an advancing (successful) sacrifice accrues to him; or he may set them up under the second (Uttara-phalgunîs)--whereby a progressive (uttarâvat) improvement accrues to him.