Shatapatha Brahmana 11.2.6.11

11.2.6.11

tā ekaviṃśatirāhutayaḥ dvādaśa vai māsāḥ saṃvatsarasya pañcartavastrayo lokāstadviṃśatireṣa evaikaviṃśo ya eṣa tapati saiṣā gatireṣā pratiṣṭhā tadetāṃ gatimetām pratiṣṭhāṃ gacati

These are twenty-one offerings,--there are twelve months and five seasons in a year; and three worlds--that makes twenty; and yonder burning (sun) is the twenty-first--that is the goal, that the resting-place: he thus reaches that goal, that resting-place.