Śatapatha Brāhmaṇa ("of a hundred paths") 4.5.3.5

4.5.3.5

taṃ vā anuṣṭubhā gṛhṇāti | gāyatraṃ vai prātaḥsavanaṃ traiṣṭubham mādhyandinaṃ savanaṃ jāgataṃ tṛtīyasavanamathātiriktānuṣṭubatyevainametadrecayati tasmādanuṣṭubhā gṛhṇāti

He draws it with an Anushtubh verse; for the morning press-feast belongs to the Gâyatrî, the midday feast to the Trishtubh, and the evening feast to the Gagatî. The Anushtubh, then, is over and above (ati-rikta), and he thus makes that (Soma of the Shodasin) to remain over: hence he takes it with an Anushtubh.