(a) Al-Hasan (may Allah have mercy on him) narrates that the Prophet (peace and blessings be upon him) said: "When a man sneezes while the imam is delivering the sermon on Friday, respond to him."
This is mursal.
(b) This is also narrated from al-Hasan as his own statement; from Salim bin Abdullah regarding returning the greeting of salam; and from Ibrahim al-Nakha'i regarding responding to the sneezer and returning the salam, and it is also narrated from him that he disliked it. It is mentioned that Sa'id bin al-Musayyib (may Allah have mercy on him) said regarding the salam: "He should return it within himself," and when he was asked about responding to the sneezer, he forbade it. And Ibn Sirin (may Allah have mercy on him), regarding the salam, used to return it by gesture and would not speak.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5848