“..this phrase ‘Ramadan Kareem’ is not correct, and the only phrase that should be said is ‘Ramadan Mubarak’ or what resembles that. Because Ramadan is not the one that gives such that it can be called generous (Kareem), rather it is only Allaah the Exalted that put virtue in it and made it a virtuous month and made the time period for performing (the fasting) a pillar from Islam. ” 

[Taken from: فتاوى العثيمين - المجلد العشرون ص (93 ) ]