Yahya related to me from Malik that Safwan ibn Sulaym said, "Ifanyone omits jumua three times without reason or illness, Allah willset a seal on his heart." (Malik said, "I don't know if it was fromthe Messenger of Allah, may Allah bless him and grant him peace, ornot.")
USC-MSA web (English) reference: Book 5, Hadith 22
Arabic reference: Book 5, Hadith 245