German defender says Sahin deserves big praise for his outstanding season with Dortmund
Sahin, 22, has agreed to join the Spanish club on a six-year deal, with Madrid reportedly paying the German side around €10 million for his services.
Hummels has said the move proves Borussia are a club worthy of producing some of the finest players in the world.
Writing on his Facebook page Hummels said: "I hope you all say goodbye to Nuri on Saturday in a way he deserves it. Also with the knowledge, that we all couldn't have become German champions if he wouldn't have played like this.
"You've got the chance to show that Borussia Dortmund is a special club. One where effort gets honoured and where a 'Dortmunder Junge' deserves the biggest compliments."
Sahin played for Dortmund for 10 years and helped them to a Bundesliga title this term.