It says in Shemos [18/6] ויאמר אל משה אני חותנך יתרו בא אליך. He said to Moshe, I Yisro your "shver" am coming to you.
In the Mechilta it says that he WROTE it in a letter. The Ramban learns this way as well. We see that even though the pasuk says that he "said", it means that he WROTE.
There is a HUUUUGE argument if writing is considered as speaking - כתיבה כדיבור [FAMOUS FAMOUS teshuva of Rebbe Akiva Eiger].
What is the argument? We have an explicit pasuk here?
