Manava Dharmashastra 2.209

2.209

abhyaktasyodvartanam utsādanaṃ na kuryāt | guruputrasya | pādayoścāvanejanaṃ prakṣālanam | asmādeva pratiṣedhādgurāvetadanuktamapi kartavyatayā pratīyate | yadā tu guruputra eva guruḥ sampadyate kṛtsnavedādhyāpanopayogitayā, tadā svanimittaṃ tatrocchiṣṭabhojanādyasti ; tadanena na pratiṣidhyate | ātideśikasyānyena niṣedho naupadeśikasya |

Medhātithi’s commentary (manubhāṣya):
‘Shampooing’—rubbing after oiling—he shall not do.

‘Washing of his feet’ also he shall not do;—for the teacher’s son.

It is from this prohibition that it follows that, even though not directly enjoined, these acts should be done for the teacher. When however the teacher’s son himself becomes the teacher, by becoming fully equipped to teach the entire Veda,—then the eating of the food left by him, etc., come to be done for his own sake; and the present prohibition does not apply to the acts under such circumstances. For their prohibition refers to what is due to the son, through the injunction transferring to him the treatment accorded to the teacher,—and not to what would be due to him by direct injunction.—(209)