Once for all, dear Most-righteous, you may do or leave undone what you think good, but please don't ask for approbation or consent to the abandonment of a principle (as for instance, the epistle to the Theatre-Intendants, and Walküre fragments). Grateful shall we be to you for all you undertake, however, since we know well that it comes hard enough to yourself to depart from principles, and you only do it with a view to furthering the cause; moreover your successes have been great and unexpected enough, to permit you unconditionally to follow your feeling. Only, as said, do not ask for the master's assent, as he stands or falls with his principles (the concerts, though they brought in much, were yet perhaps of evil); his thanks you have for everything.
Source: Letters of Richard Wagner to Emil Heckel, with a Brief History of the Bayreuth Festivals, trans. William Ashton Ellis (Grant Richards, 1899), No. 44