Uttara-vibhaga Chapter 12 Verse 13

उपासने गुरूणां च संध्ययोः साधुसंगमे। उपवीती भवेन्नित्यं विधिरेष सनातनः।।

upāsane gurūṇāṃ ca saṃdhyayoḥ sādhusaṃgame| upavītī bhavennityaṃ vidhireṣa sanātanaḥ||