Praise be to Allah.

Firstly:

Offering du‘aa’ for the deceased after the burial is prescribed in Islam and may be done by his son and others, because of the hadeeth of ‘Uthmaan ibn ‘Affaan (may Allah be pleased with him) who said: When the Prophet (blessings and peace of Allah be upon him) had finished burying a deceased person, he would stand over him and say: “Pray for forgiveness for your brother, and ask that he be made steadfast, for he is being questioned now.”

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

It is mustahabb for every Muslim to attend the burial and offer du‘aa’ for the deceased.

Secondly:

The du‘aa’ of a righteous daughter for her father is included in the words of the Prophet (blessings and peace of Allah be upon him): “When a man dies, all his good deeds come to an end except three: ongoing charity (sadaqah jaariyah), beneficial knowledge, or a righteous child who will pray for him.” Narrated by Muslim (1631) from Abu Hurayrah (may Allah be pleased with him). In Arabic, the word walad (translated here as “child”; the plural is awlaad) includes both male and female, as in the verse in which Allah, may He be exalted, says: “Allah commands you as regards your children’s [awlaadikum] (inheritance); to the male, a portion equal to that of two females” [an-Nisa’ 4:10].

Thirdly:

Congregational du‘aa’ after the burial, if it happens occasionally and does not become a regular practice, or if one of them offers du‘aa’ and the others say Ameen, is regarded as permissible by some of the scholars.

But if they always do it this way every time there is a funeral or they visit the deceased, or they single out a specific time to gather, or they recite du‘aa’ in unison, then this is a kind of bid‘ah and innovation.

Shaykh Ibn Baaz (may Allah have mercy on him) was asked: I see some people standing at the grave after burial of the deceased and offering du‘aa’ for him. Is this permissible? And is there a prescribed du‘aa’ to be said after completion of the burial? Is the du‘aa’ to be offered in congregation, such as if one person says du‘aa’ and the others say Ameen to his du‘aa’? Or should each person offer du‘aa’ on his own? Please advise us, may Allah reward you with good.

Answer: The Sunnah that is proven from the Messenger (blessings and peace of Allah be upon him) indicates that it is prescribed to offer du‘aa’ for the deceased after burial. When the Prophet (blessings and peace of Allah be upon him) had finished burying a deceased person, he would stand over him and say: “Pray for forgiveness for your brother, and ask that he be made steadfast, for he is being questioned now.” There is nothing wrong with one person saying du‘aa’ and the others saying Ameen, or each person offering du‘aa’ by himself for the deceased. And Allah is the source of strength.

End quote from Fataawa ash-Shaykh Ibn Baaz, 13/204

Shaykh Ibn Baaz (may Allah have mercy on him) was also asked:

What is the ruling on offering congregational du‘aa’ at graves?

He replied:

There is nothing wrong with it. If one person says du‘aa’ and the others say Ameen, there is nothing wrong with that, so long as it is not done deliberately and it just so happens that they hear one of them saying du‘aa’ and the others say Ameen. In such cases it is not called “congregational” because it is not done deliberately.

End quote from Fataawa ash-Shaykh Ibn Baaz, 13/340

Fourthly:

It is not prescribed to pay the imam for offering du‘aa’ for the deceased, and it is not Sunnah to stand for a long time at the grave. What good can result from the du‘aa’ of a man hired for that purpose?!

See also the answer to question no. 83829

Fifthly:

Poor imams should be helped from zakaah and charity funds, and it is not permissible to encourage them in innovations.

And Allah knows best.