Kaushitakagrihyasutra 2.7.21

2.7.21

trirātraṃ samidādhānaṃ bhaikṣācaraṇamadhaḥśayyāṃ guruśuśrūṣāṃ ca kurvan vāgyato 'pramatto 'raṇye devakule 'gnihotra upavasakha iti \21\

[says:] “For three nights, putting fuel on the fire, begging food, sleeping on the ground, and serving the guru, restrained in speech, heedful, fasting in the forest, in a temple, or at the Agnihotra, dwell.”