Мельница мелет - мука будет, язык мелет - беда будет

A man who talks much cannot but say things that can do harm. See Язык мой - враг мой (Я)

Cf: The mill that is always going grinds coarse and fine (Br.). An ox is taken by the horns, and a man by his tongue (Am., Br.). Tongue double brings trouble (Am.)