Luke 15: 3 - 7
3 So he told them this parable:
4 "What man of you, having a hundred sheep, if he has lost one of them, does not leave the ninety-nine in the wilderness, and go after the one which is lost, until he finds it?
5 And when he has found it, he lays it on his shoulders, rejoicing.
6 And when he comes home, he calls together his friends and his neighbors, saying to them, `Rejoice with me, for I have found my sheep which was lost.'
7 Just so, I tell you, there will be more joy in heaven over one sinner who repents than over ninety-nine righteous persons who need no repentance.

EWTN