口说无凭（口説無憑）　

◎ 口说无凭 kǒushuō-wúpíng

[oral expressions can not be taken as evidence;words of mouth alone don‘t carry conviction] 空口所言,无凭无据

放你不值什么,只不知你谢我多少?况且口说无凭,写一张文契才算。——《红楼梦》