Anas ibn Malik reported: Fifty prayers were obligated upon the Prophet, peace and blessings be upon him, when he was taken on his night journey. Then, it was reduced until it was set at five and an announcement was made, “O Muhammad, My word is never changed, so these five prayers will be counted as fifty.”
Source: Sunan al-Tirmidhī 213
Grade: Sahih (authentic) according to Al-Tirmidhi
عَنْ أَنَسِ بْنِ مَالِكٍ قَالَ فُرِضَتْ عَلَى النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ لَيْلَةَ أُسْرِيَ بِهِ الصَّلَوَاتُ خَمْسِينَ ثُمَّ نُقِصَتْ حَتَّى جُعِلَتْ خَمْسًا ثُمَّ نُودِيَ يَا مُحَمَّدُ إِنَّهُ لَا يُبَدَّلُ الْقَوْلُ لَدَيَّ وَإِنَّ لَكَ بِهَذِهِ الْخَمْسِ خَمْسِينَ
213 سنن الترمذي كتاب الصلاة باب كم فرض الله على عباده من الصلوات
213 المحدث الترمذي خلاصة حكم المحدث حسن صحيح في سنن الترمذي