What is the “difficulty” which makes it permissible to pray sitting down?

In the answer to
question no 50684 we have stated
that standing is a pillar or essential part of the obligatory prayers, so
the prayer of one who prays sitting when he is able to stand is not valid.
But like all other obligatory duties, this may be waived in the case of
inability to do it. 

Al-Nawawi said in al-Majmoo’ (4/201): 

The ummah is unanimously agreed that whoever is unable to
stand in an obligatory prayer may pray sitting down, and he does not have to
repeat the prayer. Our companions said: his reward will not be less than the
reward for offering the prayer standing up, because he is excused. It was
proven in Saheeh al-Bukhaari that the Messenger of Allaah (peace
and blessings of Allaah be upon him) said: “If a person is sick or is
travelling, the reward of what he used to do when he was healthy and not
travelling with be written for him.”  

The guidelines for the excuse which allows one not to stand
and to offer obligatory prayers sitting down, are as follows: 

1-    
That he be unable to stand

2-    
That it will make his sickness
worse

3-    
That it will delay his recovery

4-    
That it should cause great
difficulty of such a scale that he will be unable to focus properly. If the
difficulty caused is less than that, then it is not permissible for him to
sit. 

Al-Bukhaari (1117) narrated that ‘Imraan ibn Husayn (may
Allaah be pleased with him) said: I had haemorrhoids and I asked the Prophet
(peace and blessings of Allaah be upon him) about praying. He said:
“Pray standing, and if you cannot then sitting, and if you cannot, then
lying on your side.” 

Al-Haafiz said: The words “If you cannot” were quoted as
evidence by those who say that a sick person should not start to sit whilst
praying until after he becomes unable to stand. This was narrated by ‘Iyaad
from al-Shaafa’i. It was narrated from Maalik, Ahmad and Ishaaq that it is
not essential that a person be unable to stand, rather if it is very
difficult for him to stand (then he may sit). 

The well-known view among the Shaafa’is is that what is meant
by not being able is that it is very difficult to stand, or there is the
fear that the sickness may be made worse or it may lead to the death of the
patient. Slight difficulty is not sufficient (as an excuse). Another example
of great hardship is dizziness in the case of a passenger on a boat who
fears that he may drown if he prays standing in the boat. 

The majority also quote
as evidence the hadeeth of Ibn ‘Abbaas which was narrated by al-Tabaraani:
“He should pray standing, but if he encounters difficulty then sitting, and
if he encounters difficulty, then lying down.” 

End quote from Fath al-Baari. 

The hadeeth of Ibn ‘Abbaas which was mentioned by al-Haafiz
was also quoted by al-Haythami in Majma’ al-Zawaa’id, 2897, where he
says: 

This was narrated by al-Tabaraani in al-Awsat. He
said: It was not narrated from Ibn Jurayj except by Halas ibn Muhammad
al-Dab’i. I (al-Haythami) say: I cannot find anyone who wrote his biography,
but the rest of the men (in its isnaad) are thiqaat (trustworthy). End
quote. 

Ibn Qudaamah said in al-Mughni, 1/443: 

If he is able to stand
(he should do so), otherwise if he fears that his sickness will become worse
or his recovery will be slowed or it is extremely difficult for him to
stand, then he may pray sitting down. Something similar was said by Maalik
and Ishaaq… because Allaah says (interpretation of the meaning): 

“and has not laid upon you in religion any hardship”

[al-Hajj 22:78] 

The obligation of standing in this case is a hardship. 

And the Prophet (peace and blessings of Allaah be upon
him) prayed sitting down when he was wounded in his right cheek; it seems
that he was not completely unable to stand, but because it was difficult for
him to stand, he was excused.  End quote. 

Our companions said: Being unable does not necessarily means
that one cannot stand and slight difficulty is not enough reason for not
standing; rather what counts is obvious difficulty. If a person fears
immense hardship or worsening of the sickness etc, or if a passenger in a
boat fears that he may drown or become dizzy, he may pray sitting down, and
he does not have to repeat the prayer. Imam al-Haramayn said: What I think
with regard to the guidelines on what constitutes an inability to stand is
that standing would cause hardship that would prevent him from focusing
properly, because proper focus is the purpose of prayer. End quote. 

The view favoured by Imam al-Haramayn is the view which
Shaykh Ibn ‘Uthaymeen thought most likely to be correct. He said:  

The guideline on what constitutes hardship is that which
prevents one from focusing properly. Proper focus means having presence of
mind and being at ease. If standing will cause intense anxiety which makes
one ill at ease and makes him wish to reach the end of al-Faatihah quickly
so that he can bow, because it is too hard to stand, then he may pray
sitting down. End quote from al-Sharh al-Mumti’, 4/326.