Śatapatha Brāhmaṇa ("of a hundred paths") 1.3.5.11

1.3.5.11

ekaviṃśatiṃ sāmidhenī | api darśapūrṇamāsayoranubrūyādityāhurdvādaśa vai māsāḥ saṃvatsarasya pañcartavastrayo lokāstadviṃśatireṣa evaikaviṃśo ya eṣa tapati saiṣā gatireṣā pratiṣṭhā tadetāṃ gatimetām pratiṣṭhāṃ gacati tasmādekaviṃśatimanubrūyāt

Some people say: 'Let him recite twenty-one sâmidhenî verses also at the full- and new-moon sacrifice.' Twelve, doubtless, are the months of the year, five the seasons, and three these worlds: this (makes) twenty; and the twenty-first is this very (sun) that here shines: he is the resort, he the stay; thereby he (the sacrificer) obtains this resort, this stay. He may therefore recite twenty-one.