דברים כד:ג הלכה: Shulchan Arukh, Even HaEzer

When he gives her the divorce document, he should say to her, "Behold you are divorced from me with this divorce document and permitted to [remarry] any other," or "Behold you are sent away from me." If his messenger gives it to her, he says, "Behold you are divorced from so-and-so with this divorce document, and behold you are permitted to [remarry] any other." If he says [instead], "I am not your spouse," or "I am not your husband," some say that is not a divorce, even if he discussed with her the process of the divorce, as it says (Deuteronomy 24:1, Deuteronomy 24:3) "And he sent her away", and not that he sent himself away. And some say that if he discussed the process of divorce with her, it is a divorce of doubtful [legality].