The Book of Salah
5/3. Chapter:
How The Salah Was Made Obligatory
Sunan an-Nasai
أَخْبَرَنَا مُحَمَّدُ بْنُ هَاشِمٍ الْبَعْلَبَكِّيُّ، قَالَ أَنْبَأَنَا الْوَلِيدُ، قَالَ أَخْبَرَنِي أَبُو عَمْرٍو يَعْنِي الأَوْزَاعِيَّ، أَنَّهُ سَأَلَ الزُّهْرِيَّ عَنْ صَلاَةِ، رَسُولِ اللَّهِ صلى الله عليه وسلم بِمَكَّةَ قَبْلَ الْهِجْرَةِ إِلَى الْمَدِينَةِ قَالَ أَخْبَرَنِي عُرْوَةُ عَنْ عَائِشَةَ قَالَتْ فَرَضَ اللَّهُ عَزَّ وَجَلَّ الصَّلاَةَ عَلَى رَسُولِهِ صلى الله عليه وسلم أَوَّلَ مَا فَرَضَهَا رَكْعَتَيْنِ رَكْعَتَيْنِ ثُمَّ أُتِمَّتْ فِي الْحَضَرِ أَرْبَعًا وَأُقِرَّتْ صَلاَةُ السَّفَرِ عَلَى الْفَرِيضَةِ الأُولَى .
Abu 'Amr - meaning, Al-Awza'i - said that he asked Az-Zuhri about the prayer of the Messenger of Allah (ﷺ) in Makkah before the Hijrah to Al-Madinah. He said: "Urwah told me that 'Aishah said: 'Allah enjoined the salah upon the Messenger of Allah (ﷺ), and the first thing that He enjoined was two Rak'ahs at a time, then it was made complete four Rak'ahs while in the state of residence but the prayer when traveling remained two Rak'ahs, as it was first enjoined.'"
Reference : Sunan an-Nasa'i 454In-book reference : Book 5, Hadith 7English translation : Vol. 1, Book 5, Hadith 455