It wasnarrated that Anas bin Malik said:
"The Messenger of Allah (ﷺ)said: 'Allah enjoined fifty prayers upon my nation, and I came backwith that until I came to Musa. Musa said: 'What has your Lordenjoined upon your nation?' I said: 'He has enjoined fiftyprayers on me.' He said: 'Go back to your Lord, for your nationwill not be able to do that.' So I went back to my Lord, and Hereduced 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 dothat.' So I went back to my Lord, and He said: 'They are five andthey are fifty; My Word does not change.' So I went back to Musaand he said: 'Go back to your Lord.' I said: 'I feel shy beforemy Lord.'"
English reference: Vol. 1, Book 5, Hadith 1399
Arabic reference: Book 5, Hadith 1463