Malachi 2:14

Yet you ask, “Why?” The Lord is testifying against you on behalf of the wife you married when you were young, ¹  to whom you have become unfaithful even though she is your companion and wife by law. ² 

Ge 2:18; Ge 31:50; Jud 11:10; 1Sa 12:5; Pr 2:17; Pr 5:18,19; Pr 30:20; Ec 9:9; So 1:15; Isa 54:6; Isa 58:3; Jer 8:12; Jer 42:5; Eze 16:8; Mic 1:2; Mal 1:6,7; Mal 2:15; Mal 3:5; Mal 3:8

¹ tn Heb “the Lord is a witness between you and [between] the wife of your youth.”

² sn Though there is no explicit reference to marriage vows in the OT (but see Job 7:13; Prov 2:17; Ezek 16:8), the term law (Heb “covenant”) here asserts that such vows or agreements must have existed. References to divorce documents (e.g., Deut 24:1-3; Jer 3:8) also presuppose the existence of marriage documents.