Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.064A.

After the burial, it is desirable to pray for forgiveness of the deceased and acceptance (of his conduct by Allah), because at this time he is being questioned about his life. It is reported by ‘Uthman: “After burial the Prophet, peace be upon him, would stand by the grave of the deceased and say: “Seek forgiveness for your brother and pray for his acceptance, because he is now being questioned about it.” (This hadith is reported by Abu Daw’ud and al-Hakim, who considers it a sound hadith. Al-Bazzar says: “This is the only report from the Prophet (peace be upon him) on this subject.) Ruzain reported: “After the deceased was buried ‘Ali used to pray: ‘O Allah! This is Your servant, who is now a guest of Yours, and You are the best host. Forgive him, and expand the entrance into heaven for him.” Ibn ‘Umar liked to recite the first and the last few verses of Surah AlBaqarah by the grave after the burial was over. (Reported by Al-Baihaqi with a sound chain of narrators.)