Когда лошадь бежит, не надобно шпор

A good worker needs no driving on. See На послушного коня кнута не надо (H)

Cf: The beast that goes always never wants blows (Br.). Don't spur a willing horse (Am., Br.). Don't whip the horse that is pulling (Am.). A good horse should be seldom spurred (Am., Br.). A running horse needs no spur (Br.)