As advised by doctor my father is not allowed to put pressure on joints and hence he just do sajdah by tilting down from the chair as he perform namaz while sitting on the chair. I want to ask is it acceptable or is it mandate to perform sajdah on some hard thing say by keeping a stool in front of his chair and performing sajdah on that stool.
بسم الله الرحمن الرحيم
(Fatwa: 9/15/H=01/1440)
If it is not possible for your father to offer salah sitting on ground in any posture or he feels extreme pain then it is correct for him to offer salah while sitting on chair. There is no need to put stool etc to do sajdah on it, rather just he should bow for sajdah.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband