Sunan Ibn Majah, Chapter: The Chapters of Establishing the Prayer and the Sunnah Regarding Them

"The Messenger of Allah (saw) said: 'Allah enjoined fifty prayers upon my nation, and I came back with that until I came to Musa. Musa said: 'What has your Lord enjoined upon your nation?' I said: 'He has enjoined fifty prayers on me.' He said: 'Go back to your Lord, for your nation will not be able to do that.' So I went back to my Lord, and He reduced it by half. I went back to Musa and told him, and he said: 'Go back to your Lord, for your nation will not be able to do that.' So I went back to my Lord, and He said: 'They are five and they are fifty; My Word does not change.' So I went back to Musa and he said: 'Go back to your Lord.' I said: 'I feel shy before my Lord.'"