Offering the funeral prayer over the grave when visiting it

If you already offered the funeral prayer for your father
with the people, there is no need to repeat the prayer; rather you should
visit his grave and offer supplication for him only; you should go to the
graveyard and greet the occupants of the graves with salaam, and offer
supplication for them and for your father, as the Prophet (blessings and
peace of Allah be upon him) said: “Visit the graves, for they will remind
you of the Hereafter.” 

The Prophet (blessings and peace of Allah be upon him) used
to teach his Companions, when they visited graves, to say: Al-salaamu
‘alaykum ahl il-diyaar min al- Muslimeen wa’l-mu’mineen, wa innaa in sha
Allaah bikum laahiqoon, yarham Allaah al-mustaqdimeena minna
wa’l-musta’khireen, nas’al Allaah lana wa lakum al-‘aafiyah (Peace be
upon you, O occupants of the graves, Muslims and believers. Verily we will,
in sha Allaah, join you. May Allaah have mercy upon those who have gone
ahead of us and those who will come later on. We ask Allaah for well-being
for us and for you).” This is the Sunnah. 

So you may greet the occupants of the graves and your father,
and pray (du‘aa’) for forgiveness and mercy for him; there is no need to
offer the funeral prayer. This applies if you did offer the funeral prayer
for him. 

But if you did not offer the funeral prayer for him with the
people, you may go to his grave and offer the funeral prayer for him within
one month or less, if it is one month or less since he died. But if a long
time has passed, one should not offer the funeral prayer, according to a
number of scholars. Supplication (du‘aa’), praying for forgiveness for your
father, asking for mercy for him and giving charity on his behalf are all
acts that will benefit the deceased, whether he is a father or anyone else. 

With regard to the Mushaf that the deceased left behind, it
will benefit him if he left it as a waqf (endowment), as the reward for it
will benefit him. Similarly if he gave as a waqf books of beneficial
knowledge, Islamic knowledge or any permissible branch of knowledge or
science from which the people can benefit, he would be rewarded for that,
because it is helping in something good. Another example is if he left some
land or a house or a shop as a waqf, for charity to be given to the poor or
donated to the mosque by its income. For all of these things he would be
rewarded. The Prophet (blessings and peace of Allah be upon him) said
according to the saheeh hadeeth: “When a man dies, all his good deeds come
to an end except three: ongoing charity, beneficial knowledge, and a
righteous son who will pray for him.”  

Ongoing charity will benefit the deceased if he was a Muslim,
and he will also be benefited by the supplication of his children and
others, and by the waqf that he set up for charitable purposes such as a
house, land, a shop, date palms and so on. He will benefit from this waqf if
people benefit from it, if they eat from its income and benefit from its
income, or its income is spent on maintaining and furnishing mosques for the
Muslims. End quote. 

Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allah have mercy on him).