Shatapatha Brahmana 4.3.2.9

4.3.2.9

tayaiva tṛtīyasavanamatanvata | tāṃ ha jagatyuvācopa tvāhamāyānyekenākṣareṇopa sā hvayasva mā mā yajñādantargā iti tatheti tāmupāhvayata tato dvādaśākṣarā jagatyabhavattasmādāhurjāgataṃ tṛtīyasavanamiti

With her (Gâyatrî) indeed they performed the evening feast. Gagatî then said to her, 'To thee will I come with one syllable: invite me, and exclude me not from the sacrifice!'--'So be it!' she said and invited her. Thus the Gagatî came to consist of twelve syllables; and therefore they say, 'The evening Soma feast pertains to Gagatî.'