Mishnayos Brachos Perek 2 Mishnah 7

Another exceptional incident is related: And when his slave, Tavi, died, Rabban Gamliel accepted condolences for his death as one would for a close family member. His students said to him: Have you not taught us, our teacher, that one does not accept condolences for the death of slaves? Rabban Gamliel said to his students: My slave, Tavi, is not like all the rest of the slaves, he was virtuous and it is appropriate to accord him the same respect accorded to a family member.

וּכְשֶׁמֵּת טָבִי עַבְדּוֹ, קִבֵּל עָלָיו תַּנְחוּמִין. אָמְרוּ לוֹ תַּלְמִידָיו, לֹא לִמַּדְתָּנוּ רַבֵּנוּ, שֶׁאֵין מְקַבְּלִין תַּנְחוּמִין עַל הָעֲבָדִים. אָמַר לָהֶם, אֵין טָבִי עַבְדִּי כִּשְׁאָר כָּל הָעֲבָדִים, כָּשֵׁר הָיָה: