Hadith.1319 - Sa'id ibn al-Musayyib asked Ali ibn al-Hussain عليه السلام, "When was the prayer made obligatory upon the Muslims in the form it is today?" He عليه السلام replied: "In Medina, when the call (to Islam) became manifest, Islam was strengthened, and Allah سبحانه وتعالى, the Mighty and Majestic, prescribed jihad upon the Muslims. The Messenger of Allah صلى الله عليه وعلى آله وسلم added seven rak'ahs to the prayers: two to Dhuhr, two to Asr, one to Maghrib, and two to the Isha prayer, while leaving the Fajr prayer as it had been prescribed in Mecca. This was to hasten the ascent of the night’s angels to the heavens and the descent of the day’s angels to the earth. Thus, the angels of the night and the angels of the day would witness the Fajr prayer with the Messenger of Allah صلى الله عليه وعلى آله وسلم. This is why Allah سبحانه وتعالى, Blessed and Exalted, said: 'Recite the Qur'an at dawn. Indeed, the recitation of the Qur'an at dawn is ever witnessed' (Qur'an 17:78) — witnessed by the Muslims and by the angels of the night and the angels of the day."