Mark 10:44

10:44 and whoever wants to be first among you must be the slave of all.

Mark 13:16

13:16 The one in the field must not turn back to get his cloak.

tn Though δοῦλος (doulos) is normally translated “servant,” the word does not bear the connotation of a free individual serving another. BDAG notes that “‘servant’ for ‘slave’ is largely confined to Biblical transl. and early American times…in normal usage at the present time the two words are carefully distinguished” (BDAG 260 s.v. 1). The most accurate translation is “bondservant” (sometimes found in the ASV for δοῦλος), in that it often indicates one who sells himself into slavery to another. But as this is archaic, few today understand its force.