Śatapatha Brāhmaṇa ("of a hundred paths") 2.3.1.2

2.3.1.2

sa yatsāyamastamite juhoti | ya idaṃ tasminniha sati juhavānītyatha yatprātaranudite juhoti ya idaṃ tasminniha sati juhavānīti tasmādvai sūryo 'gnihotramityāhuḥ

When he offers in the evening after sunset, he does so thinking, 'I will offer, while he is here, who is this (offering);' and when he offers in the morning before sunrise, he does so thinking, 'I will offer, while he is here, who is this (offering):' and for this reason, they say, the Agnihotra is the Sun.