I mean really. How is a person new electronics going to make sense of an answer that says "use a JFET". ...and would you ever recommend telling anyone to buy or even build Constant Current Source for a simple LED. The LED being used by the OP still has not been explained.
The original poster is merely asking will 5V hurt my LED. The correct answer is: Your LED will be safe if you correctly limit the current.
Sadly, nobody explained HOW that current is limited based on application of OHMS law prior to DHENRY's response.
R=V/I or resistance you need is Voltage divided by Current. So, you have 5V divided by ~20mA (typical current of a standard LED)
This is a basic simple calculation to get you in the right "area", a more accurate calculation can be achieved taking into account forward voltage drop of the LED.
So. 5/.025 = 250. Closest Resistor value = 220 So, to drive a "normal/standard" 5MM LED from an Arduino pin, you insert a 220 Ohm resistor in the path from the Pin to the LED.
If you want to have some fun... go here
http://led.linear1.org/led.wizIf you want to know more. Google OHMS LAW.