Midrasch zu Schemot 18:27

(Exod. 18:5–6:) NOW JETHRO, MOSES' FATHER-IN-LAW CAME…. AND HE SAID UNTO MOSES: I AM YOUR FATHER-IN-LAW…. Who said to Moses: I AM YOUR FATHER-IN-LAW? He was unable to come to Moses. What did he do? He wrote a letter and tied it on an arrow. Then he shot, and the arrow came to Moses. Moses read it and went out to him, as stated (in vs. 7): SO MOSES WENT OUT TO MEET JETHRO. He entered the camp along with him. What is written (in vs. 12)? AND AARON CAME WITH ALL THE ELDERS OF ISRAEL TO EAT BREAD <BEFORE THE LORD WITH MOSES' FATHER-IN-LAW >. When they had eaten with him, he said: Give me permission to go to my <own> place. Moses said to him (in Numb. 10:32): IT SHALL BE THAT, IF YOU COME WITH US, IT SHALL COME TO PASS THAT WHATEVER GOOD THE LORD DOES FOR US WE WILL DO FOR YOU. <Jethro> said to him: I want to go and utter the Holy One's praise in my own place. Immediately (in Exod. 18:27): MOSES SENT HIS FATHER-IN-LAW AWAY…. Now when the Holy One does miracles for Israel, they praise him [and the nations of the world praise him]. Thus it is stated (in Ps. 138:4): {THESE PEOPLES} ALL THE KINGS OF THE EARTH SHALL GIVE THANKS TO YOU, [O LORD,] FOR THEY HAVE HEARD THE WORDS OF YOUR MOUTH.

(Exod. 19:1:) ON THE THIRD NEW MOON. What is written above on the matter? The parashah about Jethro, in which he taught Moses (in Exod. 18:21): YOU SHALL ALSO SEEK OUT <ABLE MEN> FROM AMONG ALL THE PEOPLE.³²PRK 12:16. And after that (in vs. 27): THEN MOSES SENT HIS < FATHER-IN-LAW > AWAY. And after that (in Exod. 19:1): ON THE THIRD NEW MOON. Solomon said (in Prov. 14:10): THE HEART KNOWS ITS OWN BITTERNESS…, and therefore (according to the rest of the verse) NO OUTSIDER SHALL INTERVENE IN ITS JOY. The Holy One said: My children were enslaved with clay and bricks while Jethro was dwelling quietly within his house. So shall he come to behold the joy of the Torah along with my children? Then after that (in Exod. 19:1): ON THE THIRD NEW MOON. Why so? Moses interpreted <the matter on the principle of > qal wahomer (i.e., a fortiori): If in the case of a single precept, when the Holy One was going to give the precept on Passover, Torah said (in Exod. 12:43): NO FOREIGNER SHALL EAT OF IT; in the case of six hundred and thirteen commandments which he wanted to give to Israel, should Jethro be there to behold them? Therefore (in Exod. 18:27): MOSES SENT HIS <FATHER-IN-LAW> AWAY; and after that (in Exod. 19:1): ON THE THIRD NEW MOON. Why ON THE THIRD NEW MOON, and not on the second new moon, or the seventh new moon, or on another new moon?³³PRK 12:17. Our masters have said: R. Osha'ya said: R. Hiyya the elder taught me: A female proselyte, a female captive, or an emancipated female slave may neither marry nor be betrothed for three new moons.³⁴Yev. 35a (bar); Ket. 37a (bar). So <it was with> Israel. They were called proselytes (gerim), as stated (in Lev. 19:34): FOR YOU WERE STRANGERS (gerim) IN THE LAND OF EGYPT. <They were called> captives, as stated (in Is. 14:2): THEY SHALL TAKE THEIR CAPTORS CAPTIVE. <They were called> emancipated slaves, as stated (in Lev. 26:13): I AM THE LORD YOUR GOD WHO BROUGHT YOU OUT OF THE LAND OF EGYPT TO BE THEIR SLAVES NO MORE…. The Holy One said: I will wait three new moons for them and after that I will give them the Torah.