Fiqh-us-Sunnah Volume 4, Funerals and Dhikr, Fiqh 4.088.
Section : Intention as a Prerequisite.
The performer must have the intention of performing an act on behalf of the deceased. Ibn ‘Aqil said, “If one performed any act of obedience, for example, a prayer, fasting, or recitation of the Qur’an and made its reward a gift to a deceased Muslim, the deceased will receive the reward for it, provided that he has a prior intention of making it for the benefit of the deceased, and the act and the intention go together.” Ibn al-Qayyim supports this opinion.