Manu's Code of Law 2.199 (tr. Patrick Olivelle):
Even out of sight, he must not refer to his teacher by just his name or mimic
his walk, speech, or mannerisms.
"By just his name," i.e. without an honorific title. See the translation of the same passage by Wendy Doniger and Brian K. Smith, with their note:
He should not utter his
(guru's) mere name alone even when he is out of sight, nor imitate
his gait, speech, and movements.
[199] The commentators say that he should not mention his guru's name without adding a
term of respect.
Cf.
Maimonides,
Mishneh Torah, I (
The Book of Knowledge),
Study of the Torah 5.5 (tr. Moses Hyamson):
A disciple is forbidden to call his teacher by name, even when the latter is not present.