Дал слово, держись, а не дал - крепись

You must keep your word if you cannot refrain from promising something. See Уговор дороже денег (У), Языком не спеши, а делом не ленись (Я)

Var.: Давши слово, держись, а не давши, крепись

Cf: Be slow to make a promise, but swift to keep it (Am.). Be slow to promise and quick to perform (Am., Br.). An honest man's word is as good as his bond (Am., Br.). Perform whatever you promise (Am.). A promise is a debt (a promise) (Am., Br.). A promise made is a debt unpaid (Am.). Words bind men (Br.)