(1) باب كَيْفَ فُرِضَتِ الصَّلاَةُ فِي الإِسْرَاءِ
Narrated `Aisha:
the mother of believers: Allah enjoined the prayer when He enjoined it, it was two rak`at only (in every prayer) both when in residence or on journey. Then the prayers offered on journey remained the same, but (the rak`at of) the prayers for non-travelers were increased.
Reference: Sahih al-Bukhari 350
In-book reference: Book 8, Hadith 2
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 8, Hadith 346