Actually, it is not in the PHB. It is a variant in the DMG on page 28 under Fumbles.
Specifically, "If you want to model the chance that in combat a character could fumble his weapon, then when a player rolls a 1 on his attack roll, have him make a DC 10 Dexterity check. If he fails, his character fumbles. You need to decide what it means to fumble, but in general, that character should probably lose a turn of activity as he regains his balance, picks up a dropped weapon, clears his head, steadies himself, or whatever."
Most people interpret this, or implement it, as you lose the rest of your turn when using the full attack action, which is a logical extension "lose a turn of activity", which is what we use in are group in addition to some specific thing occurring that I as DM decide on, whether it is dropping their weapon, slipping, etc.