Shatapatha Brahmana 4.1.2.20

4.1.2.20

sa yadvā upāṃśu mantreṇa juhoti | tadevāsyaiṣo 'pi mantreṇa huto bhavati kimu tattūṣṇīṃ juhuyātsamānaṃ hyetadyadupāṃśvantaryāmau prāṇodānau hi

But, as he offers the Upâmsu with a formula, even thereby this (libation) also comes to be offered with a formula. How then could one offer it silently, for these two, the Upâmsu and Antaryâma, are one and the same, since they are the out-breathing and in-breathing?