how sharper than a serpent's tooth who said it

The phrase "How sharper than a serpent's tooth it is to have a thankless child" was said by King Lear in William Shakespeare's play King Lear. This line is spoken by Lear in Act 1, Scene 4, as he expresses the deep pain caused by his ungrateful daughter, Goneril. Lear compares the emotional pain of having an ungrateful child to the sharp, painful bite of a snake, emphasizing how deeply hurt he feels by her betrayal and ingratitude after he has given up his power to his daughters.