dvitīyaṁ prāpyānupūrvyāj janmopanayanaṁ dvijaḥ
dvitīyaṁ prāpyānupūrvyāj janmopanayanaṁ dvijaḥ |
vasan guru-kule dānto brahmādhīyīta cāhutaḥ ||
(Śrīmad Bhāgavatam: 11.17.22)
“After receiving in sequence the second birth, the upanayana [-saṁskāra], and being invited [by a guru], a twice-born (dvija) should dwell in the house of the guru, be disciplined, and study the Veda.”