Prayer: The Eldest Son's Obligation for a Deceased Father's Qadaʾ Prayers
Practical Laws of Islam as per the teachings of Ayatullah Sistani
Ruling 1370
If one’s father is a believer who has not performed his daily and other obligatory prayers – excluding those prayers that had become obligatory at a specific time on account of a vow – and he could have made them up, in the event that he did not fail to perform them due to outright disobedience, then based on obligatory precaution, after the father’s death his eldest son must either perform them himself or hire someone to perform them. However, if his father intentionally did not perform them, it is not obligatory for his eldest son to make them up. The qaḍāʾ prayers of one’s mother are not obligatory for him to perform, although it is better that he does.