Bhagavatapurana 12.6.61

12.6.61

vaiśampāyana-śiṣyā vai carakādhvaryavo 'bhavan /
yac cerur brahma-hatyāṃhaḥ kṣapaṇaṃ sva-guror vratam //

61. As is well known, the disciples of Vaisampayana were Charakadhvaryavas; they were called Charakas, because they observed a vow (penance) which their preceptor had to observe and which was in expiation of the sin of Brahmahatya.