Which episode does Rachel sings with finns disabled friend?

You're thinking of the episode "The Substitute" (Season 1, Episode 11) of Glee. In this episode, Rachel sings "Mr. Cellophane" from the musical "Chicago" with Finn's disabled friend, Puck.

It's a powerful moment where Rachel, who is initially dismissive of Puck, realizes the depth of his talent and the struggle he faces.