Sunan Ibn Majah, Volume 2, Book 5, Hadith 1400 (DarusSalam) ⓘ
It was narrated that Ibn ‘Abbas said: “Your Prophet was enjoined to do fifty prayers but he returned to your Lord to make (i.e., reduce) them to five prayers.”
حَدَّثَنَا أَبُو بَكْرِ بْنُ خَلاَّدٍ الْبَاهِلِيُّ، حَدَّثَنَا أَبُو الْوَلِيدِ، حَدَّثَنَا شَرِيكٌ، عَنْ عَبْدِ اللَّهِ بْنِ عُصْمٍ أَبِي عُلْوَانَ، عَنِ ابْنِ عَبَّاسٍ، قَالَ أُمِرَ نَبِيُّكُمْ ـ صلى الله عليه وسلم ـ بِخَمْسِينَ صَلاَةً فَنَازَلَ رَبَّكُمْ أَنْ يَجْعَلَهَا خَمْسَ صَلَوَاتٍ .
also referenced under: Hadith 1400 (Reference) & Book 5, Hadith 1464 (In-Book)