Of the most rewarded sunnah is that of funeral prayer. Narrated Abu Huraira: Allah's Messenger (PBUH) said, "(A believer) who accompanies the funeral procession of a Muslim out of sincere faith and hoping to attain Allah's reward and remains with it till the funeral prayer is offered and the burial ceremonies are over, he will return with a reward of two Qirats. Each Qirat is like the size of the (Mount) Uhud. He who offers the funeral prayer only and returns before the burial, will return with the reward of one Qirat only." Al-Bukhari

This great reward astonished the companions of the prophet (PBUH) to the extent that `Amir ibn Sa`d ibn Abi-Waqas related that he was with `Abdullah ibn `Umar and Khabbah came out from his tent and addressed him saying: have you knew that  Abu-Hurairah says that he heard the Prophet (PBUH) as saying: (A believer who accompanies the funeral procession of a Muslim out of sincere faith and hoping to attain Allah's reward and remains with it till the funeral prayer is offered and the burial ceremonies are over, he will return with a reward of two Qirats. Each Qirat is like the size of the (Mount) Uhud. He who offers the funeral prayer only and returns before the burial, will return with the reward of one Qirat only.). `Abudullah ibn `Umar sent Khabbab to `Aishah, may Allah be pleased with them all, to ask her about the authenticity of this Hadith. Ibn `Umar grasped a handful of gravel overturning them at his hand in wait to the return of Khabbab who came back and said: `Aishah said: Abu-Hurairah is truthful in saying that. Ibn `Umar threw the gravel to the ground and said: We missed too many Qirats. Narrated by Muslim.

Ibn Umar regretted for the reward he missed by not waiting until burial, because he has been used to observe funeral prayers and do not wait until it is buried which is demonstrated by the narration of Muslim on the authority of Salim ibn Abdullah who said: ibn Umar used to perform funeral prayers and then leaves, but when he knew the Hadith, he said: we missed too many Qirats.

This sunnah has many advantages:

- A believer performing funeral prayer finds warning of death and be rewarded by one or two Qirats at paradise.

- The deceased attains forgiveness for supplications invoked for him.

- Family of the deceased find support by others participation

http://books.islamstory.com/en/artical/3407700/80- Sunnah of funeral prayer