Praise be to Allah.
The Sunnah for the one who wants to offer supplication for the deceased after he has been buried and the soil has been levelled over him is to offer supplication whilst standing. The basic principle concerning that is the report narrated by Abu Dawood with his isnaad from ‘Uthmaan (may Allaah be pleased with him) who said: When the Prophet (blessings and peace of Allaah be upon him) finished burying the deceased person, he would stand over him and say: “Ask for forgiveness for your brother and ask that he be made steadfast, for right now he is being questioned.” Abu Dawood and al-Mundhiri remained silent about it. It was also narrated by al-Haakim who classed it as saheeh, and by al-Bazzaar who said: It was not narrated from the Prophet (blessings and peace of Allaah be upon him) except through this isnaad.
And Allaah is the source of strength. They Allaah send blessings and peace upon our Prophet Muhammad and his family and Companions. End quote.