When is it permissible for the Imam or the congregant to pray while sitting, and what are the rulings regarding that?

If the imam of the mosque is unable to perform the pillars of prayer (such as standing, bowing, and prostrating), he should appoint someone else to lead the prayer, to avoid the scholarly dispute regarding the validity of a disabled person leading an able-bodied congregation. If he leads them while sitting, then the congregants pray sitting from the beginning of the prayer according to the Hanbalis, and they pray standing absolutely according to the Shafi'is. The manner of sitting in prayer for one who is unable to stand is commendable: the worshipper sits cross-legged during the standing posture, and folds his legs when bowing and prostrating, and he may sit in any other posture. Sitting during obligatory prayers is prescribed when one is unable to stand, or fears an increase in illness, or a delay in recovery, or experiences unbearable hardship.