Hadith 503370
Chapter 9: Shortening the Prayer - كتاب قصر الصلاة فى السفر
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ صَالِحِ بْنِ كَيْسَانَ، عَنْ عُرْوَةَ بْنِ الزُّبَيْرِ، عَنْ عَائِشَةَ، زَوْجِ النَّبِيِّ صلى الله عليه وسلم أَنَّهَا قَالَتْ فُرِضَتِ الصَّلاَةُ رَكْعَتَيْنِ رَكْعَتَيْنِ فِي الْحَضَرِ وَالسَّفَرِ فَأُقِرَّتْ صَلاَةُ السَّفَرِ وَزِيدَ فِي صَلاَةِ الْحَضَرِ .
Yahya related to me from Malik from Salih ibn Kaysan from Urwa ibn az-Zubayr that A'isha, the wife of the Prophet, may Allah bless him and grant him peace, said, "The prayer was prescribed as two rakas, both when settled and when travelling. Then the travelling prayer was kept as it was, and an increase was made in the prayer when settled. "
Reference Muwatta Malik 503370
USC-MSA web (English) reference : Book 9, Hadith 9