The Lubavitcher Rebbe, in one of his letters (letter no. 184, dated from 7 Shvat, 5705), cites the Rebbe Rayatz (The Tzemach Tzedek) and says:
A word spoken is public knowledge; in writing, it is for the entire world, and in print, it is for generations after generations.’”
The Rebbe quotes the Tzemach Tzedek from Igros Kodesh (אגרות קודש) of the Rebbe Rayatz, Vol. II, Letter No. 558
Does anyone know where to find this? I could not find it unfortunately