Narrated 'Aishah (RA) that when the Salat (prayer) was first prescribed, it consisted of two Rak'at. Afterwards, the prayer during travelling was confirmed (as two Rak'at), while the prayer at the place of residence was completed (as four Rak'at). [Agreed upon].

عَنْ عَائِشَةَ رَضِيَ اَللَّهُ عَنْهَا قَالَتْ: { أَوَّلُ مَا فُرِضَتْ اَلصَّلَاةُ رَكْعَتَيْنِ , فَأُقِرَّتْ صَلَاةُ اَلسَّفَرِ وَأُتِمَّتْ صَلَاةُ اَلْحَضَرِ } مُتَّفَقٌ عَلَيْه ِ [1]‏ .‏