Bulugh al-Maram 2 The Book of Prayer كتاب الصلاة 332

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).

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].