The following is a summary translation from a post taken from Sahab.net
Questioner: 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?
Sheik Uthaymeen: said:
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. And it is as though the one who said this thinks that due to the nobility of this month it is permissible to commit sins.
And that is in opposition to what the people of knowledge have said (for they have said) that the sins are multiplied if they are done during virtuous times or noble places so this is the opposite of what this person has pictured. And they have said that it is incumbent upon the person to have Taqwaa of ALLAH the Mighty and Majestic during every time and in every place especially during virtuous times and in noble places. And ALLAH the Mighty and Majestic says “Oh you who believe fasting has been prescribed for you like it was prescribed for those before you that you may obtain Taqwaa.”
So the wisdom behind the obligation of fasting is to gain Taqwaa of ALLAH the Mighty and Majestic by doing what He has ordered and avoiding what He has prohibited. And it has been established that the Prophet peace and blessing be upon him said “Whoever does not abandon falsehood in word and action, then Allah Mighty and Majestic has no need that he should leave his food and drink”. Therefore fasting is worship for ALLAH and cultivation for the soul and a safeguard for it from the prohibitions of ALLAH. And it is not like this one without knowledge has said that due to the nobility of this month and its blessing, sinning is allowed in it.
The More We Learn, We Realise The Less We Know - Imam Shafi