Tirmidhi

1:
That is:
Initially, fifty daily prayers had been made obligatory, but although they were reduced to five daily prayers, the reward remains that of fifty prayers.
Moreover, with Allah, the reward for every good deed has, from the outset, been counted as tenfold.

450. Commentary:

➊ "A portion of the prayer was forgiven": In Arabic, the word «شطر» is used, which means both "half" and "a portion," hence the latter meaning has been adopted. There is also brevity in this narration; otherwise, the prayers were reduced in sets of five.

➋ In «لَا يُبَدَّلُ الْقَوْلُ لَدَيَّ», "statement" refers to the spoken word, i.e., the statement regarding fifty prayers—that despite the reduction, their reward remained intact.

➌ The Ibn Hazm mentioned in the chain refers to Abu Bakr bin Muhammad bin Amr bin Hazm. He is counted among the Tabi‘in (successors).

Benefits and Issues:

➊
This hadith narrates a part of the incident of Mi'raj (the Night Journey and Ascension).
For details, see: (Sahih al-Bukhari, The Book of Prayer, Chapter: How the Prayer Was Made Obligatory During the Night Journey, Hadith: 349)

➋
When Musa (alayhis salam) said that your ummah will not be able to bear so many prayers,
the reason for this was that he had such an experience with Bani Isra'il.
Bani Isra'il had shown negligence in performing prayers as commanded by Allah. (Sahih Muslim, The Book of Faith, Chapter: The Night Journey of the Messenger of Allah sallallahu alayhi wa sallam to the Heavens and the Obligation of Prayers, Hadith: 162)

➌
The command being changed from fifty prayers to five is a special mercy from Allah Ta'ala and a tremendous favor upon the Muslims. The only way to show gratitude for this favor is to perform all five prayers regularly, with full observance of their etiquettes, and at their prescribed times.

➍
By declaring the five prayers to be equivalent to fifty, it was said:
“My word does not change.”
The reason for this is that it is His own law that a good deed performed correctly and with sincerity is recorded at least tenfold in reward.
It is stated:
﴿مَن جَاءَ بِالْحَسَنَةِ فَلَهُ عَشْرُ أَمْثَالِهَا﴾ (al-An'am: 160)
Whoever brings a good deed shall have ten times as much (reward).

➎
On the last occasion, the Messenger of Allah sallallahu alayhi wa sallam refrained from requesting further reduction,
because in the glad tidings of the reward of fifty for five, it was stated that now no further reduction would be made.