The Rabbis taught: Four [Sages] entered the Pardes [literally "the orchard." Rashi explains that they ascended to heaven by utilizing the [Divine] Name, i.e., they achieved a spiritual elevation (Tosafot, ad loc) through intense meditation on G-d's Name]. They were Ben Azzai, Ben Zoma, Acher [Elisha ben Avuya, called Acher - the other one - because of what happened to him after he entered the Pardes] and Rabbi Akiva. Rabbi Akiva said to them [prior to their ascension]: "When you come to the place of pure marble stones, do not say, 'Water! Water!' for it is said, 'He who speaks untruths shall not stand before My eyes' (Psalms 101:7)." Ben Azzai gazed [at the Divine Presence - Rashi] and died. Regarding him the verse states, "Precious in the eyes of G-d is the death of His pious ones" (Psalms 116:15). Ben Zoma gazed and was harmed [he lost his sanity - Rashi]. Regarding him the verse states, "Did you find honey? Eat as only much as you need, lest you be overfilled and vomit it up" (Proverbs 25:16). Acher cut down the plantings [he became a heretic]. Rabbi Akiva entered in peace and left in peace.

Source: http://www.ascent.org.il/NewAscentOfSafed/Teachings/
Advanced/Kabbala/pardes.html

Four Entered the Orchard

Our rabbis taught:
Four (men) entered into the Garden [Pardes] --
(Simeon) ben 'Azzai and (Simeon) ben Zoma, the apostate [acher] and Rabbi Aqiba.
Rabbi Aqiba said to them:
-- "When you arrive at the stones of pure marble, do not say:
-- 'Water! Water!' For it is said:
-- "He who tells lies shall not stand before my eyes" (Ps. 101.7b).
(Simeon) ben 'Azzai looked and died; of him scripture says:
-- "Precious in the eyes of the LORD is the death of his saints" (Ps. 116:15)
(Simeon) ben Zoma looked and was stricken (insane); of him scripture says:
-- "Have you found honey? Eat only what you can hold, lest you be stuffed with it and spit it out" (Prov 25:16). The apostate cut down his own offspring [lit.: "shoots"].
Rabbi Aqiba departed in peace.

Source: Babylonian Talmud Hagiga 14b.