In an opinion piece which appeared in the AMS Notices of January 2010, John Wermer tells us that he once heard about a seminar given by Grothendieck which was described as "a telegram by Grothendieck to Serre". Is this anecdote recorded somewhere?
I think I heard Grothendieck's talk at the 1958 ICM described as a "a telegram by Grothendieck to Serre". But this would have been in a conversation (with neither Serre nor Grothendieck). I don't know whether anyone said it in a lecture, much less wrote it down.
MR0130879 (24 #A733) Grothendieck, Alexander The cohomology theory of abstract algebraic varieties. 1960 Proc. Internat. Congress Math. (Edinburgh, 1958) pp. 103--118 Cambridge Univ. Press, New York
Added: I don't know who first described Grothendieck's 1958 ICM talk as a "telegram to Serre", but I probably heard it from Lubin. Since Lubin and the article's author spent their careers at Brown, this fits.
Grothendieck's talk is available here. In it he lays out his plans for schemes and their cohomology. In 1958, not many people would have understood it.
You might want to know that a book exists, which consists of correspondences between Serre and Grothendieck. Its Google-Books link is this.