The Peach Blossom villagers told him it was 'not to be spoken of to outsiders.'
Tips
history
From Tao Yuanming's (陶渊明, Eastern Jin dynasty) 《桃花源记》: 此中人语云:不足为外人道也 — 'A villager said to him: this is not to be told to outsiders.' The parting request from the hidden utopia of the Peach Blossom Spring to the fisherman who stumbled in. After he leaves, no one ever finds it again.
usage
为 is read wèi (4th tone, 'for / to'), not wéi (2nd tone, 'is'). 也 is a classical sentence-final particle — no modern meaning, just formality. Used today as an educated way to say 'keep this between us.'