Śatapatha Brāhmaṇa ("of a hundred paths") 2.3.1.36

2.3.1.36

tadāhuḥ | agnāvevaitatsāyaṃ sūryaṃ juhoti sūrye prātaragnimiti tadvai taduditahomināmeva yadā hyeva sūryo 'stametyathāgnirjyotiryadā sūrya udetyatha sūryo jyotirnāsya sā paricakṣeyameva paricakṣā yattasyai nāddhā devatāyai hūyate yāgnihotrasya devatāgnirjyotirjyotiragniḥ svāheti tatra nāgnaye svāhetyatha prātaḥ sūryo jyotijyotiḥ sūryaḥ svāheti tatra na sūryāya svāheti

They also say, 'In the evening he offers Sûrya in Agni, and in the morning he offers Agni in Sûrya.' Such, indeed, is the case with those who offer after sunrise; for when the sun sets then Agni is the light, and when the sun rises then Sûrya is the light. Here no offence is committed on his (the sacrificer's) part; but an offence is indeed committed where offering is not made distinctly to that deity (viz. Agni or Sûrya resp.), which is the deity of the Agnihotra. He says, 'Agni is the light, the light is Agni, Svâhâ!' and not, 'To Agni Svâhâ!' and in the morning, 'Sûrya is the light, the light is Sûrya, Svâhâ!' and not, 'To Sûrya Svâhâ!'