who said how sharper than a serpent's tooth

The phrase "How sharper than a serpent's tooth" was said by King Lear in William Shakespeare's play King Lear. The full line is "How sharper than a serpent's tooth it is to have a thankless child!" This quote appears in Act 1, Scene 4 and expresses King Lear's deep pain and bitterness about the ingratitude of his daughter Goneril, comparing the emotional wound it causes to the sharp bite of a serpent.