口ㄎㄡˇkǒu說ㄕㄨㄛshuō無ㄨˊwú憑ㄆㄧㄥˊpíng

光靠嘴說不足為證。《初刻拍案驚奇·卷一○》：「設誓到也不必，只是口說無憑，請朝奉先回。」《紅樓夢·第一二回》：「況且口說無憑，寫一張文契才算。」反一言為定、有案可稽

客口說無憑

英(idiom) you can't rely on a verbal agreement, just because sb says sth, doesn't mean it's true