Originally Posted by Umm ul-Shaheed
I'm trying to think what I would do in the above scenario. The one thing that comes to mind is that I wouldn't immediately use the word "wrong". I'd more likely start off with something positive, like, "I loved the way you washed your arms so thoroughly maashaAllah!" and then say something along the lines of..."Did you know that rasoolullah sallallaahu alaihi wa sallam actually used to/didn't used to do such and such in his wudhu?" Depending on what's deficient/or added to the wudhu, you can say that "Doing such and such is not mentioned in the hadeeth, or according to hadeeth you also have to do such and such. I'm hoping my memory serves me correctly about that, I'll see if I can find the hadeeth for you, and it'll be a good learning exercise for me too, as it'll refresh my memory. Alhamdulillah we'll both learn something new and improve our deen. Rasoolullah wanted us to do wudhu his way, and you Alhamdulillah are the person who has spurred me on to find that hadeeth on what his way was. May Allah reward you for that abundantly."
That way you have whats known as the yes-no-yes sandwich, used when you need to correct a person and there's a risk that they might feel they are being criticised, and might not do the correction you suggest. Start off with and end with a positive, with what you actually want to say in the middle. Reinforce in the closing positive, that you'll find the hadeeth, and thank them for being the person that reminded you. When you show them the hadeeth use that opportunity then to reinforce the correction of what they had done wrong.
Even if you know that what you're saying is correct you can use that excuse to actually produce the hadeeth if possible, and hopefully
the other person won't feel that they are being found fault with.
I think there'll probably still be a few people who might take offence, no matter how nicely you try to say it, but you can only do your best. Even if they do take offence, and don't listen, Allah will know that you tried and reward you accordingly!