Praying behind an incapacitated imam

It is permissible for the imam to lead the people in prayer in the manner mentioned in the question, and it is permissible for him to lead the people in prayer whilst sitting down. This is the view of Abu Haneefah, al-Shaafa’i and Ahmad. 

How the worshippers behind him should pray depends on the situation: 

1 – If the imam starts praying sitting down, then the people behind him should pray sitting down. 

The evidence for this is as follows: 

(i)It was narrated that ‘Aa’ishah the Mother of the Believers said: “The Messenger of Allaah (peace and blessings of Allaah be upon him) prayed at home when he was sick. He prayed sitting down and some people prayed behind him standing up, but he gestured to them that they should sit. When he finished (the prayer) he said, ‘The imam has been appointed to be followed. When he bows, bow; when he raises his head, raise your heads; and if he prays sitting, then pray sitting.’” (Narrated by al-Bukhaari, al-Adhaan, 657)

(ii)It was narrated from Anas ibn Maalik that the Messenger of Allaah (peace and blessings of Allaah be upon him) rode a horse and fell, and the right side (of his body) was injured. He offered one of the prayers sitting down, and we prayed behind him sitting down.  When he finished (the prayer) he said, ‘The imam has been appointed to be followed. If he prays standing up, then pray standing up; when he bows, bow; when he raises his head, raise your heads. When he says Sami’ Allaahu liman hamidah (Allaah hears the one who praises Him), say Rabbanaa wa laka’l-hamd (Our Lord, to You be praise). If he prays standing, then pray standing, and if he prays sitting, then pray sitting, all of you.’” (Narrated by al-Bukhaari, al-Adhaan, 648).

2 – If the imam starts the prayer standing, then something happens to him that makes him unable to stand any more, those who are praying behind him should complete the prayer standing. 

The evidence for this is as follows: 

(i)It was narrated that ‘Aa’ishah (may Allaah be pleased with her) said: “The Prophet (peace and blessings of Allaah be upon him) became seriously ill and asked, ‘Have the people prayed?’ We replied, ‘No, they are waiting for you.’ He said, ‘Put some water in a tub for me.’ We did that and he took a bath and tried to get up, but he fainted. Then he (peace and blessings of Allaah be upon him) recovered, and again asked, ‘Have the people prayed?’ We said, ‘No, they are waiting for you, O Messenger of Allaah.’ He again said, ‘Put some water in a tub for me.’ He sat down and took a bath and tried to get up but fainted again. Then he recovered and said, ‘Have the people prayed?’ We said, ‘No, they are waiting for you, O Messenger of Allaah.’ He again said, ‘Put some water in a tub for me.’ He sat down and took a bath and tried to get up but fainted again. Then he recovered and said, ‘Have the people prayed?’ We said, ‘No, they are waiting for you, O Messenger of Allaah.’ The people were in the mosque, waiting for the Prophet (peace and blessings of Allaah be upon him) to lead them in ‘Isha prayer. The Prophet (peace and blessings of Allaah be upon him) sent word to Abu Bakr that he should lead the people in prayer. The messenger went to him and said, ‘The Messenger of Allaah (peace and blessings of Allaah be upon him) is telling you to lead the people in prayer.’ Abu Bakr, who was a soft-hearted man, said: ‘O ‘Umar, lead the people in prayer.’ But ‘Umar answered, you have more right to do that.’ So Abu Bakr led the prayers during those days. Then when the Prophet (peace and blessings of Allaah be upon him) felt a little better, he came out supported by two men, one of whom was al-‘Abbaas, for the Zuhr prayer. Abu Bakr was leading the people in prayer, but when Abu Bakr saw him, he wanted to step back [to let the Prophet (peace and blessings of Allaah be upon him) lead the prayer]. But the Prophet (peace and blessings of Allaah be upon him) gestured to him that he should not step back. He said, ‘Sit me down beside him,’ so they sat him down beside Abu Bakr. Abu Bakr continued praying, but he followed the Prophet (peace and blessings of Allaah be upon him) in prayer, and the people were following Abu Bakr in the prayer, and the Prophet (peace and blessings of Allaah be upon him) was sitting down.” (narrated by al-Bukhaari, al-Adhaan, 655).

The relevant point in this hadeeth is that Abu Bakr started to lead them in prayer standing up, then the Prophet (peace and blessings of Allaah be upon him) came and led them from where Abu Bakr had reached, and he prayed sitting down, and the Sahaabah prayed behind him standing up. This indicates that if the imam starts the prayer standing, then he becomes unable to carry on standing, then the people praying behind him should still pray standing. 

This does not mean that the prayer is lacking and it does not detract from its blessing, in sha Allaah. And Allaah knows best. 

See Ahkaam al-Imaamah wa’l-I’timaam fi’l-Salaah by al-Muneef, p. 112-116; Fath al-Baari Sharh Saheeh al-Bukhaari by Ibn Hajar, vol. 2, p. 174.