Narrated from Ya'qub ibn Yazid, from Ibn Abi Umayr, from Hisham ibn Salim, from Abu Abdullah عليه السلام, who said:
Imam عليه السلام said: "There is no harm if a person prays for the deceased after they have been buried."
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 1, p. 495