The prayers were prescribed upon the Prophet ﷺ on the night he was taken on the night journey as fifty, then they were reduced until they were made five. Then a call came, "Muhammad, the word with Me is not changed, and you shall have for these five the reward of fifty."
فرضت على النبي ﷺ ليلة أسري به الصلوات خمسين، ثم نقصت حتى جعلت خمسا، ثم نودي: يا محمد إنه لا يبدل القول لدي، وإن لك بهذه الخمس خمسين.
Tirmidhi narrated it in the Book of Prayer (213) in this wording, abridged, from Muhammad bin Nisaburi, who said, Abdul-Razzaq narrated to us, from Maamar, from Zuhri, from Anas. He said, Hasan sahih gharib. It is a long hadith that will come later in the story of the Night Journey and Ascension (Isra and Miraj), and this portion was also narrated by Bukhari in the Book of the Beginning of Creation (3207), from Saeed and Hisham, who both said, Qatada narrated to us, Anas narrated to us, and by Muslim in the Book of Faith (163) through the route of Yunus, from Ibn Shihab, from Anas.