On one occasion Ven. Sabhiya Kaccana was staying at Ñatika in the Brick Hall. Then Vacchagotta the wanderer went to him and, on arrival, exchanged courteous greetings with him. After an exchange of friendly greetings & courtesies, he sat to one side. As he was sitting there, he said to Ven. Sabhiya Kaccana, "Now then, Master Kaccana, does the Tathagata exist after death?"
"Vaccha, that has not been declared by the Blessed One: 'The Tathagata exists after death.'"
"Well then, Master Kaccana, does the Tathagata not exist after death?"
"Vaccha, that too has not been declared by the Blessed One: 'The Tathagata does not exist after death.'"
"Then does the Tathagata both exist and not exist after death?"
"That has not been declared by the Blessed One: 'The Tathagata both exists and does not exist after death.'"
"Well then, does the Tathagata neither exist nor not exist after death?"
"That too has not been declared by the Blessed One: 'The Tathagata neither exists nor does not exist after death.'"
"Now, Master Kaccana, when asked if the Tathagata exists after death, you say, 'That has not been declared by the Blessed One: "The Tathagata exists after death."' When asked if the Tathagata does not exist after death, you say, 'That too has not been declared by the Blessed One: "The Tathagata does not exist after death."' When asked if the Tathagata both exists and does not exist after death, you say, 'That has not been declared by the Blessed One: "The Tathagata both exists and does not exist after death."' When asked if the Tathagata neither exists nor does not exist after death, you say, 'That too has not been declared by the Blessed One: "The Tathagata neither exists nor does not exist after death."' Now, what is the cause, what is the reason, why that has not been declared by Gotama the contemplative?"
"Vaccha, whatever cause, whatever reason there would be for describing him as 'possessed of form' or 'formless' or 'percipient' or 'non-percipient' or 'neither percipient nor non-percipient': If that cause, that reason, were to cease totally everywhere, totally in every way without remainder, then describing him by what means would one describe him as 'possessed of form' or 'formless' or 'percipient' or 'non-percipient' or 'neither percipient nor non-percipient'?"
"How long has it been since you went forth, Master Kaccana?"
"Not long, my friend. Three years."
"Whoever has gained just this much in this much time has gained a great deal, my friend — to say nothing of the things he has thus gone beyond."
See also: other suttas in the Avyakata Samyutta (and the translator's Introduction); AN 4.42.