Maliks Muwatta Book 5, Hadith Number 22.
Section : The Recitation in the Jumua Prayer, the Sitting, and Missing the Prayer Without a Reason.
Yahya related to me from Malik that Safwan ibn Sulaym said, “If anyone omits jumua three times without reason or illness, Allah will set a seal on his heart.” (Malik said, “I don’t know if it was from the Messenger of Allah, may Allah bless him and grant him peace, or not.”)