On the prohibition against addressing an iniquitous person or innovator, as "master", etc.

Riyad as-Salihin (The Meadows of the Righteous)

by Imam Nawawi

321. Chapter: On the prohibition against addressing an iniquitous person or innovator, as "master", etc.

1725. Burayda reported that the Messenger of Allah, may Allah bless him and grant him peace, said, "Do not call the hypocrite 'master'. Even if he is a master, you will anger your Lord, the Mighty and Majestic." [Abu Dawud]