Fatwa by Shaykh Uthaymeen
When the fasting person falls into a sin and he is prohibited from that sin he will say, “Ramadan Kareem”. So what is the ruling concerning this phrase and what is the ruling concerning this behavior?
The ruling concerning this is, 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 ALLAH 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.
(more information can be found at Muttaqun online.)