Слово не горобець, вилетить — не спіймаєш.

“A word is not a sparrow — once it flies out, you won't catch it.”

Think before you speak, because once said, words cannot be taken back.

— Traditional Ukraine Wisdom, Ukraine

Source: Traditional Ukrainian proverb, public-domain oral tradition (recorded in M. Nomys, 'Українські приказки, прислів'я і таке інше', 1864)