This happens if the chuck wasn't tightened securely by hand, or you're drilling in the wrong gear.
First, for safety, turn the unit off and remove the battery pack. Now, open and close the chuck by hand. Insert the bit, then twist the chuck sleeve tightly by hand until it's firmly closed. Don't use the drill's motor to tighten it.
Second, make sure you're in the correct speed setting. For drilling, especially larger holes, you should be in low speed, high torque (position 1, selector away from the chuck). High speed (position 2) has less torque and is more for driving screws.
If that didn't work: The chuck or internal spindle might be damaged. Have the tool serviced by a qualified repair person. Do not continue using a loose chuck; the bit can fly out.
Heads up: Always secure your workpiece with clamps. Holding it by hand leaves it unstable and can lead to loss of control if the bit binds.