Śatapatha Brāhmaṇa ("of a hundred paths") 1.1.1.2

1.1.1.2

so 'gnimevābhīkṣamāṇo vratamupaiti | agne vratapate vrataṃ cariṣyāmi tacakeyaṃ tanme rādhyatāmityagnirvai devānāṃ vratapatistasmā evaitatprāha vrataṃ cariṣyāmi taccakeyaṃ tanme rādhyatāmiti nātra tirohitamivāsti

Looking towards the (Âhavanîya) fire, he enters on the vow, with the text (Vâg. S. I, 5 a): 'O Agni, Lord of Vows! I will keep the vow! may I be equal to it, may I succeed in it!' For Agni is Lord of Vows to the gods, and it is to him therefore that he addresses these words. In the words, 'I will observe the vow; may I be equal to it; may I succeed in it,' there is nothing that requires explanation.