SHAFAQNA – The Grand Ayatollah Sistani answered a question about the conditions for the son who is responsible for his father’s lapsed Salaats (obligatory Salaats which his father was not able to perform).
Question: The eldest son is responsible for the lapsed Salaats of father and mother; what is the duty of the eldest son if they did not pray (they were Tareko salaat)?
The Grand Ayatollah Sistani: The son is not responsible for mother’s lapsed Salaats, and the Salaats which the father had the excuse not to perform and (the father) despite having the ability to perform afterwards but did not perform; in this condition, to be cautious, it is Wajeb for the eldest son to perform the lapsed Salaats. This caution does not apply if the eldest son is a junior or mad at the time of his father’s death as well as if he is prohibited to inherit (from his father). But the eldest son is not responsible, if his father has not performed Salaats on purpose, although this is recommended (Mostahab).
Source: PERSIAN SHAFAQNA