Yahya related to me from Malik that he had heard that a mansneezed on the day of jumua while the imam was giving the khutba, anda man by his side asked Allah to bless him. Said ibn al Musayyab wasasked about it and he forbade the man to do what he had done and said,"Don't do it again."
USC-MSA web (English) reference: Book 5, Hadith 10
Arabic reference: Book 5, Hadith 235