بسم الله والحمد لله والصلاة والسلام على رسول الله وعلى آله وصحبه ومن تبع هداه وبعد
From the Branches of Faith
Allaamah ash-Shanawani رحمه الله تعالى said: “It is related regarding Abu Da’wud (the compiler of the Sunan), that he was on board a ship. He heard a person on shore sneezing and saying “Al Hamdulillah”. So he hired a small rowing boat for one dirham and went (a little distance) to shore and replied to the person by saying: “Yarhamukallah”.
Imam Abu Da’wud was then asked about this action of his, he answered: “It is possible that this person who sneezed is a man whose supplication is accepted by Allaah.”* – When people on-board (the ship) fell asleep, they heard a voice calling out: “O people of the ship! Surely Abu Da’wud has purchased Paradise from Allaah for one dirham!”
* He may have expected the person to reply to him by saying; “May Allah have mercy on us and you and forgive us and you.” As was the practise of our Salaf (see al-Bukhari, 6224)
[Ash-Shanawani ‘Sharh al-Mukhtasar ibn Abi Jamrah’]
1. Imam Malik related from Nafi’ that when Abdullah ibn Umar رضي الله عنه sneezed and someone said to him “Yarhamukallah” (may Allaah have mercy on you), he replied, “Yarhamunallah wa yaghfirlana wa lakum” (may Allaah have mercy on us and you, and forgive us and you). [Muwatta Imam Malik]
2. Returning the salaam and ‘tashmeet’ (asking Allaah’s Mercy for the one who sneezes) is from the branches of faith – [see Fath al-Baari, hadeeth 9]
The Prophet, صلى الله عليه وسلم, said: “Indeed Allaah loves the one who sneezes and dislikes yawning. So if someone sneezes and then praises Allah it is obligatory upon every Muslim who heard him to say, ‘May Allaah be merciful to you …[يَرْحَمُكَ الله] …’”
Shaykh Al-Albaani (may Allaah have mercy upon him) said, “And his صلى الله عليه وسلم saying, ‘… then it is obligatory upon every Muslim who heard him …’ is a clear proof showing the obligation upon everyone who heard him to say thetashmeet (i.e., may Allaah be merciful to you [يَرْحَمُكَ الله]). And what has become widespread about it only being a fard kifaayah, i.e., that when some people say it the obligation is no longer upon the rest to say it also, is something which is not proven here, in contrast to returning the salaam.” [Tahqeeq Kalimah at-Tayyib, 158]
Taken from: http://shaikhalbaani.wordpress.com
And Hidaayah is from Allaah alone.