Brewer's: Spare the Rod and Spoil the Child
Solomon (Prov. xiii. 24) says: “He that spareth the rod hateth his son;” but Samuel Butler, in his Hudibras (pt. ii. canto 1, line 843), says:
Love is a boy, by poets styled, Then spare the rod, and spoil the child.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894