What is your opinion concerning making Du`a' (supplication) for the deceased? Does it benefit the deceased?
The lawful Du`a` benefits the deceased according to the Ijma` (consensus) of Ahl-ul-Sunnah (Part No. 9; Page No. 18) wal-Jama`ah (those adhering to the Sunnah and the Muslim main body), for Allah (Exalted be He) says:
And those who came after them say: "Our Lord! Forgive us and our brethren who have preceded us in Faith, and put not in our hearts any hatred against those who have believed. Our Lord! You are indeed full of kindness, Most Merciful.
The Prophet (peace be upon him) made Du`a` for the deceased during his visit to the graves and in other situations and he taught his Sahabah (the Companions) what to say in terms of Du`a` upon visiting the graves and his saying to them when he buried a Muslim man:
Seek forgiveness for your brother, for he will be questioned now.
Furthermore, he made Du`a` for the deceased during Funeral Prayer and taught his Sahabah the supplications they should recite when praying for the deceased in addition to many other Hadiths to that effect which reach the degree of Mutawatir (a Hadith reported by a significant number of narrators throughout the chain of narration, whose agreement upon a lie is impossible) with regard to their meaning, and there is no contradiction between that and the saying of Allah (Exalted be He):
And that man can have nothing but what he does (good or bad).
The generality of this Ayah (Qur'anic verse) is specified by the Ayah regarding the day of Resurrection and many other Ayahs and Hadiths.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.