|Credit: see below.|
Above is a scan (both sides) of a letter sent by Bobby Fischer to the Israel Chess Federation, provided to us by Yoram Lubianiker. He adds that, while the date of the cancellation stamp is not clear, the stamp on the bottom right was issued in Nov. 3rd, 1970, which means that is the earliest date the letter could have been sent.
Fischer (after his participation in Netanya 1968) was invited to participate in a tournament in Israel by the ICF again, but politely declined, noting he is devoting all his energies to the conquest of the world championship. This, Lubianiker learned from person conversation from a source in the ICF which I also know, and who I know from personal experience wishes not to be named on the Internet.