"O people of the ship! Surely Abu Dawud purchased paradise from Allah for one dirham!"

Allamah ash-Shanawani said: 

It is related with regard to Abu Dawud (as-Sijistani), the author of the Sunan [Abi Dawud], that he was on board of ship. He heard a person at shore sneezing and saying “Alhamdulillah.“ [the praise is for Allah].

So he hired a rowing boat for one dirham and went to shore and replied to this person’s saying: “Yarhamukullah“ [Allah have mercy on you]. 

Imam Abu Dawud then was asked about this action of his, he answered: 

It is possible that this person who sneezed is a man whose supplication is accepted by Allah*.  

When people on board fell asleep, they heard a voice calling out:

O people of the ship! Surely Abu Dawud purchased paradise from Allah for one dirham!.

 

SOURCE: Ash-Shanawani «Sharhul mukhtasar ibn Abi Jamrah» p 290.)

*  He may have expected that person to reply to him by saying; “May Allah have mercy on us and you and forgive us and you“… As was practised by the Salaf:

Malik related to me from Nafi that when Abdullah ibn Umar sneezed and someone said to him, “May Allah have mercy on you,” (Yarhamuka’llah), he said, “May Allah have mercy on us and you, and forgive us and you.” (Yarhamuna’llah wa yaghfirlana wa lakum).

 Malik’s Muwatta Book 54, Number 54.2.5 

LINK:  http://ahlalhdeeth.com/vbe/showthread.php?t=8428