Sunan an-Nasa'i, Hadith 455 ⓘ
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."
أَخْبَرَنَا قُتَيْبَةُ، عَنْ مَالِكٍ، عَنْ صَالِحِ بْنِ كَيْسَانَ، عَنْ عُرْوَةَ، عَنْ عَائِشَةَ، قَالَتْ فُرِضَتِ الصَّلاَةُ رَكْعَتَيْنِ رَكْعَتَيْنِ فَأُقِرَّتْ صَلاَةُ السَّفَرِ وَزِيدَ فِي صَلاَةِ الْحَضَرِ .
also referenced under: Book 5, Hadith 8 (In-Book) & Volume 1, Book 5, Hadith 456 (DarusSalam)