كتاب الصلاة
2  
The Book of Prayer
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 .‏


‏1 ‏- صحيح.‏ رواه البخاري (1090)‏، ومسلم (685)‏.‏

Sunnah.com reference : Book 2, Hadith 336
English translation : Book 2, Hadith 424
Arabic reference : Book 2, Hadith 429