What it is Sunnah to say at the burial

It is Sunnah for the one who places the deceased in the
grave to say: “Bismillaah wa ‘ala sunnati Rasool-Illaah (or: wa
‘ala millati Rasool-Illaah) sall-Allaahu ‘alayhi wa sallam (In
the name of Allaah and according to the Sunnah of the Messenger of Allaah
(or: according to the religion of the Messenger of Allaah (peace and
blessings of Allaah be upon him)). 

This is narrated in the hadeeth of Ibn ‘Umar (may Allaah be
pleased with him) who said that when the Prophet (peace and blessings
of Allaah be upon him) placed the deceased in the grave he would say (and
according to another version, he said: When you place your dead in the
graves say:) “Bismillaah wa ‘ala sunnati Rasool-Illaah”; according to
another report: “wa ‘ala millati Rasool-Illaah.” 

This was narrated by Abu Dawood, 3213; al-Tirmidhi, 1046; Ibn
Maajah, 1550; al-Haakim, 1353; classed as saheeh by al-Albaani in Ahkaam
al-Janaa’iz, p. 192. 

After completion of the
burial it is Sunnah to pray for forgiveness for the deceased and pray for
him to be made steadfast, because of the hadeeth of ‘Uthmaan ibn ‘Affaan
(may Allaah be pleased with him) who said: When the Prophet (peace and
blessings of Allaah be upon him) had finished burying the deceased he would
stand over him and say: “Pray for forgiveness for your brother, and ask that
he be made steadfast, for now he is being questioned.” 

Narrated by Abu Dawood, 3221; classed as saheeh by al-Albaani
in Ahkaam al-Janaa’iz, p. 198. 

Standing over him means
standing over his grave. 

“and [said]: Pray for forgiveness for your brother” means:
ask for forgiveness for him from Allaah, i.e., say: “O Allaah forgive him.” 

“and ask that he be made steadfast” means: ask Allaah to make
his tongue steadfast in answering the two angels, i.e., say: “O Allaah make
him steadfast in speech.” 

“for now he is being questioned” i.e., at that moment the two
angels are coming to him, namely Munkar and Nakeer, so he is in the greatest
need of prayers for forgiveness and steadfastness.