This is from Oneaday.org, a sister site of Danwei’s that publishes a Chinese saying or idiom, with translation, every week day:
口说无凭立字为据
kou3 shuo1 wu2 ping2 li4 zi4 wei2 ju4
Spoken words fly away, written words remain
See this idiom on oneaday.org in simplified or traditional characters.
If you think the idiom could be better translated, please let us know in the comments section.