Do you know where that is in the Koran? I can't find it.
Here ya go...
Qur'an 3:27: "Let not the believers take the disbelievers for friends rather than believers. And whoever does this has no connection with Allah unless it is done to guard yourselves against them, guarding carefully. And Allah cautions you against His retribution. And to Allah is the eventual coming."
Qur'an 16:106: Any one who, after accepting faith in Allah, utters unbelief, except under compulsion, his heart remaining firm in Faith - but such as open their breast to Unbelief, on them is Wrath from Allah, and theirs will be a dreadful penalty.