Reb Yosef Tzvi Segal relates:
In 5724 (1964), Reb Osher Sassonkin was one of the first chassidim to escape the Iron Curtain and make his way to the Rebbe in New York.
At his first yechidus, the Rebbe began talking to him with a respectful plural, as is accepted in Yiddish. Reb Osher wouldn’t hear of it. “Rebbe, why do you talk like that to me?”
“Why, you look older than your father!” the Rebbe replied, referring to the elder chossid Reb Shmerel Sassonkin.
“Rebbe! I heard from [the mashpia] Reb Chatche with my own ears: a chossid ages in years, but not in spirit!”
The Rebbe smiled affectionately. From then on, the Rebbe spoke with him in direct terms, dropping the plural.
(Otzar Hachasidim EY page 448; Reb Chatche Feigin served as mashpia and mazkir of the Frierdiker Rebbe)