The Book of Prayer
Bulugh al-Maram #512
كتاب الصلاة
وَلِلْبُخَارِيِّ: «ثُمَّ هَاجَرَ, فَفُرِضَتْ أَرْبَعًا, وَأُقِرَّتْ صَلَاةُ اَلسَّفَرِ عَلَى اَلْأَوَّلِ»
al-Bukhari has:
'Then emigrated and it was prescribed as four, but prayer while travelling was left according to the original prescription (of two Rak'at).'
وَلِلْبُخَارِيِّ: "ثُمَّ هَاجَرَ, فَفُرِضَتْ أَرْبَعًا, وَأُقِرَّتْ صَلَاةُ اَلسَّفَرِ عَلَى اَلْأَوَّلِ"
'Then emigrated and it was prescribed as four, but prayer while travelling was left according to the original prescription (of two Rak'at).'
— Bulugh al-Maram, Hadith 512