Cheers Season 4 Episode 5

Sam: "The quality of mercy is not strained..."
Sam is quoting from "The Merchant Of Venice", the 1600 play by William Shakespeare (1564-1616). The line is from Act IV, scene 1 and is part of Portia's attempt to persuade Shylock to give up his claim of a "pound of flesh" that Antonio has offered as guarantee for Shylock's loan to Bassanio.