Shatapatha Brahmana 4.3.2.8

4.3.2.8

tayā prātaḥsavanamatanvata | tasmādgāyatraṃ prātaḥsavanaṃ tayaiva mādhyandinaṃ savanamatanvata tāṃ ha triṣṭubuvācopa tvāhamāyāni tribhirakṣarairupa mā hvayasva mā mā yajñādantargā iti tatheti tāmupāhvayata tata ekādaśākṣarā triṣṭubabhavattasmādāhustraiṣṭubham mādhyandinaṃ savanamiti

With her they performed the morning feast of the Soma-sacrifice,--whence the morning feast pertains to Gâyatrî. With her they performed the midday feast. Trishtubh then said to her, 'To thee will I come with three syllables: invite me, and exclude me not from the sacrifice!'--'So be it!' she said and invited her. Thus the Trishtubh came to consist of eleven syllables, and therefore they say, 'The midday Soma feast pertains to Trishtubh.'