Hadith 455
Chapter 3: How The Salah Was Made Obligatory - كتاب الصلاة
أَخْبَرَنَا قُتَيْبَةُ، عَنْ مَالِكٍ، عَنْ صَالِحِ بْنِ كَيْسَانَ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، قَالَتْ فُرِضَتِ الصَّلاَةُ رَكْعَتَيْنِ رَكْعَتَيْنِ فَأُقِرَّتْ صَلاَةُ السَّفَرِ وَزِيدَ فِي صَلاَةِ الْحَضَرِ .
It was narrated that 'Aishah said:
"The Salah was enjoined two Rak'ahs at a time, then the Salah when traveling remained like that, but the Salah while resident was increased."
Reference : Sunan an-Nasa'i 455
In-book reference : Book 5, Hadith 8