وَفِي دُعَاءِ الْأَحْيَاءِ لِلْأَمْوَاتِ وَصَدَقَاتِهِمْ مَنْفَعَةٌ لِلْأَمْوَاتِ
The supplications and charities of the living benefit those who have died.
Duʿāʾ includes asking for mercy and forgiveness for a deceased person. Charity includes giving on their behalf so that they benefit from its reward.
This establishes the benefit of these practices. Individual acts of worship and how their rewards may be conveyed must still be considered according to their particular evidence.