The anecdote is very widely told and 57 is called the Grothendieck prime because of it. Its literal veracity is disputed within mathematical folklore, and the tellings do not agree on the setting or the wording.
What would resolve this A first-hand account from someone present, or a contemporary written record of the exchange.
OpenHistory of mathematics, and oral history of the Grothendieck circle.Allyn Jackson, Comme Appele du Neant, As If Summoned from the Void: The Life of Alexandre Grothendieck