The casting time is 1 reaction, which you take when you see a creature within 60 feet of you casting a spell.

You attempt to interrupt a creature in the process of casting a spell. If the creature is casting a spell of 3rd level or lower, its spell fails and has no effect.

If it is casting a spell of 4th level or higher, make an ability check using your spell casting ability. The DC equals 10+ the spell's level.

On a success, the creature's spell fails and has no effect.

At Higher Levels. 

When you cast this spell using a spell slot of 4th level or higher, the interrupted spell has no effect if its level is less than or equal to the level of the spell slot you used.