What to Say after Sneezing and it's Reply

When you or someone sneezes.
(1st person) Say: 
Alhamdulillah 
All praise be to Allah.

Person beside you Must reply
(2nd person) Reply: 
Yarhamukallah 
May Allah be merciful to you.

(1st person) Then Reply: 
Yahdikumullah wa Yuslih balakum 
May Allah give you guidance and improve your condition.

Narrated Abu Huraira:
The Prophet (ﷺ) said, " If anyone of you sneezes, he should say 'Alhamdulillah' (Praise be to Allah), and his (Muslim) brother or companion should say to him, 'Yarhamukallah'(May Allah bestow his Mercy on you). When the latter says 'Yarhamukallah', the former should say, 'Yahdikumul-lah wa Yuslih balakum' (May Allah give you guidance and improve your condition).
Sahih al-Bukhari 6224

Allah Loves Sneezing but Dislikes Yawning

Narrated Abu Huraira:
The Prophet (ﷺ) said, "Allah likes sneezing and dislikes yawning, so if someone sneezes and then praises Allah, then it is obligatory on every Muslim who heard him, to say: May Allah be merciful to you Yarhamukallah'. But as regards yawning, it is from Satan, so one must try one's best to stop it, if one says 'Ha' when yawning, Satan will laugh at him."

Sahih al-Bukhari 6223