There are so many topics for firewall logging but I cannot adopt it to my problem.
I added a "Network -> Firewall -> Traffic Rule" which rejects the traffic from a specific MAC-address in lan to wan (completly).
Because i still want to enable some traffic from this rule I'd like to see what traffic is rejected (destination IP).
Pleas help me. Thank U in advance.
It's not so trivial, but also not impossible.
Thats it. Thank U. 
I just added -s for filtering SourceIP and have less loglines.
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I found an easier way to log (imo).
In Network -> Firewall -> Traffic Rules just add a new rule and in Advanced Settigs Tab enter -j LOG to Extra Arguments.
Thats it. U can easily enable/disable at this point. And U can move the rule to the right postion.