Śatapatha Brāhmaṇa ("of a hundred paths") 5.5.3.3

5.5.3.3

tasyaikaviṃśam prātaḥsavanam | saptadaśam mādhyandinaṃ savanam pañcadaśaṃ tṛtīyasavanaṃ sahokthaiḥ saha ṣoḍaśinā saha rātryā

Twenty-onefold is (each stotra of) its Morning-service, seventeenfold (of) the Midday-service, fifteen-fold (of) the Evening-service, together with the Uktha (stotras), the Shodasin, and (the twelve stotras of) the Night-service.