Sayyiduna al-Bara' bin Azib (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) commanded us seven things and forbade us seven things: he commanded "visiting the sick, attending funerals, returning the greeting of salam, accepting invitations, fulfilling oaths, responding to one who sneezes, and helping the oppressed"; and he forbade "wearing a gold ring, drinking from silver vessels, wearing every kind of silk, sitting on silk cushions, and using red silk saddle-cloths."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5846
Sayyiduna Abu Hurayrah (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) said: "Five things are the right of a Muslim over a Muslim: returning the greeting of salam, responding to one who sneezes, visiting the sick, attending the funeral, and accepting an invitation."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5847
(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