How was prayer first prescribed?

I have heard that during the early years of islam, muslims were told to perform 40 prayers a day, but this was cut down by 5 each time ALLAH (S.W.T) knew this was unachievable by majority of muslims until their were only 5 salat a day were left. is this because ALLAH (S.W.T) has shown us mercy?.

Praise be to Allaah.

What you have heard about the prayer is not correct. The correct version is that Allaah initially – on the night of the Mi’raaj when our Prophet (peace and blessings of Allaah be upon him) was taken up into heaven – enjoined fifty prayers to be performed night and day. Then the Prophet (peace and blessings of Allaah be upon him) kept asking his Lord to reduce it until it became five prayers to be offered night and day. But Allaah decreed the reward of fifty prayers for these five, so whoever prays five prayers will receive the reward of fifty prayers. 

It was narrated that Anas ibn Maalik said: Abu Dharr narrated that the Messenger of Allaah (peace and blessings of Allaah be upon him) said: “The roof of my house was split open when I was in Makkah, and Jibreel (peace and blessings of Allaah be upon him) came down and split open my chest and washed it with Zamzam water. Then he brought a golden vessel filled with wisdom and faith and poured it into my chest, then he sealed it. Then he took me by the hand and ascended with me into the lowest heaven. When I reached the lowest heaven Jibreel said to the gatekeeper, ‘Open up!’ He said, ‘Who is this?’ He said, ‘Jibreel.’ He said, ‘Is there anyone with you?’ He said, ‘Yes, Muhammad (peace and blessings of Allaah be upon him) is with me.’ He said, ‘Has his mission begun?’ He said, ‘Yes.’ And when it was opened we went up into the lowest heaven… Then Allaah enjoined upon my ummah fifty prayers, and I came back until I passed by Moosa who said, ‘What has Allaah enjoined upon your ummah?’ I said, ‘He has enjoined fifty prayers.’ He said, ‘Go back to your Lord, for your ummah will not be able to bear that.’ So I went back and He reduced it. Then I came back to Moosa and said, ‘He has reduced it.’ He said, ‘Go back to your Lord, for your ummah will not be able to bear that.’ So I went back and He reduced it further. Then I came back to Moosa and said, ‘He has reduced it further.’ He said, ‘Go back to your Lord, for your ummah will not be able to bear that.’ So I went back and He said, ‘(The prayers) are five but (the reward) is fifty. The Sentence that comes from Me cannot be changed [cf. Qaaf 50:29].’ Then I went back to Moosa and he said, ‘Go back to your Lord.’ But I said, ‘I feel shy before my Lord.’…” 

Narrated by al-Bukhaari, 342; Muslim, 163 

Al-Haafiz Ibn Hajar said: 

What is meant is that they are five with regard to what is to be done, but they are fifty with regard to the reward. 

Fath al-Baari, 1/463 

And Allaah knows best.