Tongues, then, are a sign, not for believers, but for unbelievers. Prophecy, however, is for believers, not for unbelievers.
KJV
Wherefore tongues are for a sign, not to them that believe, but to them that believe not: but prophesying [serveth] not for them that believe not, but for them which believe.
ASV
Wherefore tongues are for a sign, not to them that believe, but to the unbelieving: but prophesying is for a sign, not to the unbelieving, but to them that believe.