Sunan an-Nasa'i 449
Anas bin Malik and Ibn Hazm said:
"The Messenger of Allah (ﷺ) said: 'Allah, the Mighty and Sublime, enjoined fifty prayers upon my Ummah, and I came back with that until I passed by Musa, peace be upon him, who said: 'What has your Lord enjoined upon your Ummah?' I said: 'He has enjoined fifty prayers on them.' Musa said to me: 'Go back to your Lord, the Mighty and Sublime, for your Ummah will not be able to do that.' So I went back to my Lord, the Mighty and Sublime, and He reduced a portion of it. Then I came back to Musa and told him, and he said: 'Go back to you Lord, for your Ummah will not be able to do that.' So I went back to my Lord, the Mighty and Sublime, and He said: 'They are five (prayers) but they are fifty (in reward), and the Word that comes from Me cannot be changed.' [1] I came back to Musa and he said: 'Go back to your Lord.' I said: 'I feel too shy before my Lord, the Mighty and Sublime.'"

[1]See Surah Qaf 50:29.
أَخْبَرَنَا يُونُسُ بْنُ عَبْدِ الأَعْلَى، قَالَ حَدَّثَنَا ابْنُ وَهْبٍ، قَالَ أَخْبَرَنِي يُونُسُ، عَنِ ابْنِ شِهَابٍ، قَالَ أَنَسُ بْنُ مَالِكٍ وَابْنُ حَزْمٍ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏
"‏ فَرَضَ اللَّهُ عَزَّ وَجَلَّ عَلَى أُمَّتِي خَمْسِينَ صَلاَةً فَرَجَعْتُ بِذَلِكَ حَتَّى أَمُرَّ بِمُوسَى عَلَيْهِ السَّلاَمُ فَقَالَ مَا فَرَضَ رَبُّكَ عَلَى أُمَّتِكَ قُلْتُ فَرَضَ عَلَيْهِمْ خَمْسِينَ صَلاَةً ‏.‏ قَالَ لِي مُوسَى فَرَاجِعْ رَبَّكَ عَزَّ وَجَلَّ فَإِنَّ أُمَّتَكَ لاَ تُطِيقُ ذَلِكَ ‏.‏ فَرَاجَعْتُ رَبِّي عَزَّ وَجَلَّ فَوَضَعَ شَطْرَهَا فَرَجَعْتُ إِلَى مُوسَى فَأَخْبَرْتُهُ فَقَالَ رَاجِعْ رَبَّكَ فَإِنَّ أُمَّتَكَ لاَ تُطِيقُ ذَلِكَ ‏.‏ فَرَاجَعْتُ رَبِّي عَزَّ وَجَلَّ فَقَالَ هِيَ خَمْسٌ وَهِيَ خَمْسُونَ لاَ يُبَدَّلُ الْقَوْلُ لَدَىَّ ‏.‏ فَرَجَعْتُ إِلَى مُوسَى فَقَالَ رَاجِعْ رَبَّكَ فَقُلْتُ قَدِ اسْتَحْيَيْتُ مِنْ رَبِّي عَزَّ وَجَلَّ ‏"‏ ‏.‏