Jesus tells this parable of the lost coin:

He speaks of a woman that has ten silver coins, but she loses one coin. She lights a lamp, sweeps the house and looks for it very carefully. When she finds the coin, she gathers her friends and neighbors with her and they rejoice together.

Jesus went on to explain that it is the same way when one sinner repents.  The angels in the presence of God all rejoice over this one sinner that has repented.