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