Importance of dua for the deceased following burial

Translation
After the Prophet (peace and blessings of Allah be upon him) would
bury the deceased he would stand by the grave and say:
"Seek forgiveness for your brother and pray that he remains firm, for
he is now being questioned."
Sources: Abu Dawud 3/315 and Al-Hakim 1/370 who graded it authentic
and Ath-Thahabi agreed.