The Prophetic Sunnah mentions both sneezing and yawning and emphasises the importance of the sneezer praising Allah. If he/she is heard praising Allah by someone, they should respond with "YarhamukAllah" - May Allah have Mercy on you.
Yawns should be suppressed as best as possible and they should be covered with a hand over the mouth.
Sneezing and Yawning from the authentic narrations;
Allah loves sneezing and dislikes yawning. If someone yawns, he should put his hand over his mouth.
Abu Huraira (radhi Allahu anhu) narrated that the Prophet (sallAllahu alayhi wassallam) said, "Allah loves sneezing but dislikes yawning, so if anyone of you sneezes and then praises Allah, every Muslim who hears him (praising Allah) has to say Tashmit to him (respond with YarhamukAllah). But as regards yawning, it is from satan, so if one of you yawns, he should try his best to suppress it, for when anyone of you yawns, satan laughs at him."
Sahih Al Bukhari Book 78, Hadith 250.
The etiquette between the one who sneezes and his companion.
Abu Huraira (radhi Allahu anhu) reported Allah's Messenger (sallAllahu alayhi wassallam) as saying, "When one of you sneezes, he should say: "Praise be to Allah in every circumstance," and his brother or his companion should say: "May Allah have mercy on you!" And he should then reply: "May Allah guide you and set right your affairs."
Sunan Abi Dawood Book 43, Hadith 261. Sh Al Albaani (rahimullah) classed the isnad as Saheeh.
Sneezing loudly is not from the Sunnah.
Abu Huraira (radhi Allahu anhu) narrated that when the Prophet (sallAllahu alayhi wassallam) would sneeze, he would cover his face with his hand or with his garment, and muffle the sound with it.
Jami at Tirmidhi Book 43, Hadith 2969. Graded as Hasan by Darussalam.
Replying to the one who has sneezed and praised Allah is one of the rights that one Muslim has over another Muslim.
Abu Huraira (radhi Allahu anhu) reported Allah's Messenger (sallAllahu alayhi wassallam) as saying, "Six are the rights of a Muslim over another Muslim." It was said to him: Allah's Messenger, what are these? Thereupon he said, "When you meet him, offer him greetings. When he invites you to a feast, accept it. When he seeks your council, give him, and when he sneezes and says "All praise is due to Allah," you say Yarhamuk Allah (may Allah show mercy to you), and when he fails ill, visit him, and when he dies, follow his bier (funeral procession)."
Sahih Muslim Book 39, Hadith 6.