Read 1 Corinthians 14:22 Using Other Translations
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.
Thus tongues are a sign not for believers but for unbelievers, while prophecy is a sign not for unbelievers but for believers.
So you see that speaking in tongues is a sign, not for believers, but for unbelievers. Prophecy, however, is for the benefit of believers, not unbelievers.