Charles Ellicott Commentary Psalms 116:16

"O Jehovah, truly I am thy servant: I am thy servant, the son of thy handmaid; Thou hast loosed my bonds." — Psalms 116:16 (ASV)

Your servant, and the son of Your handmaid. — Compare Psalm 86:16. Not only himself but his family were in the covenant, and, as very commonly in the East, the mother is selected for mention instead of the father.