(48) باب التَّارِيخِ مِنْ أَيْنَ أَرَّخُوا التَّارِيخَ
Narrated `Aisha:
Originally, two rak`at were prescribed in every prayer. When the Prophet (ﷺ) migrated (to Medina) four rak`at were enjoined, while the journey prayer remained unchanged(i.e. two rak`at).
Reference: Sahih al-Bukhari 3935
In-book reference: Book 63, Hadith 161
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 5, Book 58, Hadith 272