All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.
Shykhul-Islam Ibn Taymiyyah was asked about the authenticity of a Hadeeth stating that when one says “La ilaaha illa Allaah”, i.e. none is worthy of worship but Allaah, 70,000 times and dedicates its reward to a dead person, he earns salvation from Hellfire, and whether the reward reaches the dead person. He answered, “When a person says “La ilaaha illa Allaah”, i.e. none is worthy of worship but Allaah, 70,000 times, less, or more and dedicates its reward to a dead person, Allaah will benefit the dead person with this action; however, there is no authentic or weak Hadeeth to this effect.” [Majmoo‘ Al-Fataawa, 24/324]
In the previous question, Ibn Taymiyyah underlined the difference in opinions among Muslim jurists regarding the ruling on dedicating the reward of righteous deeds to a dead person. He then commented, “However, this was not the practice of the righteous predecessors. Instead, when they performed a voluntary prayer, a fast, Hajj or recited the Quran, they did not dedicate the reward of these deeds to the dead Muslims in general or to some of them in particular. Thus, one should not abandon the way of the righteous predecessors but should follow their example, because it is better and more perfect.”
Ibn Taymiyyah was referring to his statement before that in which he said, "The first Muslim generations used to perform all kinds of righteous deeds prescribed by the Sharee'ah and carry out obligatory and voluntary acts of worship including prayer, fasting, recitation of the Quran, Thikr and the like and they used to supplicate for the believers as enjoined by Allaah and supplicate Allaah for the living and for the dead in the funeral prayers and when visiting the graves and on similar occasions. It was reported on the authority of a number of the righteous predecessors that the supplication upon completing the recitation the whole Quran is answered. It is permissible for a Muslim after completing the recitation the whole Quran to supplicate for himself, one's parents, one's teachers, and for the believers. The same applies to supplicating for the dead while performing night prayers and at other times when supplications are likely to be answered."
This is a brief account on Ibn Taymiyyah's opinion in this regard.
Allaah Knows best.