The concept of tidal locking appeared, at least qualititively, in proposition 38 of book III of Newton's principia. Newton wrote there:
Hence it is that the same face of the moon always respects the earth; nor can the body of the moon possibly rest in any other position, but would return always by a libratory motion to this situation; but those librations, however, must be exceedingly slow, because of the weakness of the forces which excite them; so that the face of the moon, which should be always obverted to the earth, may, for the reason assigned in Prop. XVII be turned towards the other focus of the moon's orbit, without being immediately drawn back, and converted again towards the earth.
Proposition 38 is in the subsection entitled "forces to move the sea"; this fact shows Newton placed the "tidal locking" effect within the theory of tidal forces.