Maliks Muwatta Book 5, Hadith Number 10.

Yahya related to me from Malik that he had heard that a man sneezed on the day of jumua while the imam was giving the khutba, and a man by his side asked Allah to bless him. Said ibn al Musayyab was asked about it and he forbade the man to do what he had done and said, “Don’t do it again.”