Śatapatha Brāhmaṇa ("of a hundred paths") 2.6.3.2

2.6.3.2

atha yasmācunāsīryeṇa yajete | yā vai devānāṃ śrīrāsītsākamedhairījānānāṃ vijigyānānāṃ tacunamatha yaḥ saṃvatsarasya prajitasya rasa āsīttatsīraṃ sā yā caiva devānāṃ śrīrāsītsākamedhairījānānāṃ vijigyānānāṃ ya u ca saṃvatsarasya prajitasya rasa āsīttamevaitadubhayam parigṛhyātman kurute tasmācunāsīryeṇa yajate

Then as to why he should perform the Sunâsîrya offering. The prosperity (srî) that accrued to the gods on performing the Sâkamedha offerings, and gaining the victory (over Vritra), is suna; and the essence (rasa) that belonged to the year gained by them is sîra. Now that same prosperity which accrued to the gods on performing the Sâkamedha offerings, and that same essence which belonged to the year gained by them,--both these he takes possession of and makes his own: that is why he performs the Sunâsîrya.