All the slaves who were circumcised together with our father Abraham did not endure, neither they nor their descendants, within Israel. Then why did he circumcise them? For the sake of purity, so that they would not defile their masters through their food and their drink. (Pirkei deRabbi Eliezer, chapter 29)
Why Abraham Circumcised the Slaves of His Household
Yalkut Shimoni on Torah 82:9
כָּל הָעֲבָדִים שֶׁנִּמּוֹלוּ עִם אָבִינוּ אַבְרָהָם לֹא נִתְקַיְּמוּ לֹא הֵם וְלֹא זַרְעָם בְּיִשְׂרָאֵל, וְלָמָּה מָלָן, בִּשְׁבִיל טַהֲרָה, שֶׁלֹּא יְטַמְּאוּ אֶת אֲדוֹנֵיהֶם בְּמַאֲכָלֵיהֶם וּבְמִשְׁתֵּיהֶם (פדר"א פכ"ט).