al-Bukhari has:

'Then [(Allah's Messenger (ﷺ)] 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 } [1]‏ .‏