"Alcohol makes you more likely to do things without thinking them through. This can increase your chances of slipping and smoking a cigarette."
Alcohol & Smoking | Smokefree
While I don 't agree that smoking a cigarette after you have quit is a slip (unless a wet floor, banana peel or a spot of ice is involved), this article, I think, is helpful in understanding why it's a bad idea to drink alcohol while you are quitting smoking - and to limit your alcohol consumption after your quit is well in hand.
A quit is a terrible thing to lose!
Nancy