بُلُوغُ الْمَرَامِ
Bulugh al-Maram 429
The Book of Prayer
عَنْ عَائِشَةَ رَضِيَ اَللَّهُ عَنْهَا قَالَتْ: { أَوَّلُ مَا فُرِضَتْ اَلصَّلَاةُ رَكْعَتَيْنِ , فَأُقِرَّتْ صَلَاةُ اَلسَّفَرِ وَأُتِمَّتْ صَلَاةُ اَلْحَضَرِ } مُتَّفَقٌ عَلَيْه ِ [1] .
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].
Sunnah.com reference: Book 2, Hadith 336