The classic quote on mercy is that of Portia in Shakespeare's The Merchant of Venice, Act IV, Scene I, lines 186-197.

The quality of mercy is not strain'd
It droppeth as the gentle rain from heaven
Upon the place beneath: It is twice blest;
It blesseth him that gives and him that takes:
'Tis mightiest in the mightiest: it becomes
The throned monarch better than its crown;
His scepter shows the force of temporal power,
The attribute to awe and majesty,
Wherein doth sit the dread and fear of kings;
But mercy is above this sceptred sway;
It is enthroned in the hearts of kings;
It is an attribute to God himself,·
And earthly power doth then show likest
God's When mercy seasons justice.