Famous Martin Luther Quote. Source?

Here is a famous quotation which is ascribed to Martin Luther. I have often heard it. But where is it from?

Heaven knows we Christians are often busy fighting the wrong battles.

If I profess with the loudest voice and clearest exposition every portion of the Word of God except precisely that little point which the world and the devil are at that moment attacking, I am not confessing Christ, however boldly I may be professing Him. Where the battle rages there the loyalty of the soldier is proved; and to be steady on all the battle front besides, is mere flight and disgrace if he flinches at that point.