Commentary for Exodus 18:6

ויאמר אל משה AND HE SAID UNTO MOSES through a messenger (Mekhilta d'Rabbi Yishmael 18:6).

AND HE SAID UNTO MOSES: ‘I THY FATHER-IN-LAW JETHRO AM COMING UNTO THEE.’ He sent him the message in a letter in which [the above words] were written. A messenger, [as Rashi would have it], could not say, I thy father-in-law. Instead, he would say, “Behold, Jethro your father-in-law is coming to you.” It is also not possible that Jethro told him so mouth to mouth, for in that case he would have said, “Behold, I have come to you.” Besides, it is not customary in such instances for the speaker to mention his name: “I, such and such a person,” for upon seeing him, he would recognize him. A similar case is the verse: Then Huram the King of Tyre said⁶⁴Thus the word “said” can clearly apply to “saying in writing.” in writing, which he sent to Solomon.⁶⁵II Chronicles 2:10.

אני חותנך יתרו, Yitro sent this message as a matter of courtesy so that Moses would not be taken unawares by his sudden arrival. He complied with the sages’ warning “do not enter your own house without advance notice, how much more so the house of your friend.” (Pessachim 112).