is burdensome except for those who show humble devotion outside it but not within it,” this would imply that prayer is not burdensome for those who lack humble devotion within it, but is burdensome for those who possess it. This would negate the meaning of the verse. It is therefore established that humble devotion is obligatory in prayer.

Its obligatory nature is also indicated by the words of Allah Most High:

“Successful indeed are the believers,”
“those who are humbly devout in their prayers,”
“those who turn away from idle talk,”
“those who pay zakah,”
“those who guard their chastity,”
“except with their wives or those whom their right hands possess, for then they are free from blame.”
“But whoever seeks beyond that—it is they who are the transgressors.”
“And those who faithfully observe their trusts and pledges,”
“and those who diligently maintain their prayers.”
“Those are the inheritors,”
“who will inherit Firdaws, where they will abide forever.”

He, glorified and exalted be He, states that these are the ones who will inherit Firdaws in Paradise. This implies that no others will inherit it, demonstrating that these qualities are obligatory. If any of them were merely recommended, the Garden of Firdaws could be inherited without that quality, since Paradise is attained by fulfilling obligations, without requiring the performance of recommended deeds. This is why He mentioned only obligatory qualities here.

Since humble devotion in prayer is obligatory, and humble devotion encompasses both tranquillity and humility, relevant here is the report from Umar, may Allah be pleased with him, who saw a man fidgeting during his prayer and said:

“Had this man’s heart been humbly devout, his limbs would have been so as well.”

That is, they would have become still