Śatapatha Brāhmaṇa ("of a hundred paths") 10.2.5.3

10.2.5.3

etadu ha yajñe tapaḥ yadupasadastapo vā upasadastadyattapasi cīyate tasmāttāpaścitastadvai yāvadevopasadbhiścaranti tāvatpravargyeṇa saṃvatsaramevopasadbhiścaranti saṃvatsaram pravargyeṇa

And the Upasads also are the fervour in the sacrifice, for they are indeed fervour; and inasmuch as it is built (ki) in fervour (tapas) it is called 'Tâpaskita.' As long as they perform the Upasads so long (do they perform) the Pravargya: (if) it is for a year. that they perform the Upasads, it is for a year (they perform) the Pravargya.