When the Salat (prayer) was first prescribed, it consisted of two Rakat. Afterwards, the prayer during travelling was confirmed (as two Rakat), while the prayer at the place of residence was completed (as four Rakat).
عَنْ عَائِشَةَ رَضِيَ اَللَّهُ عَنْهَا
قَالَتْ: «أَوَّلُ مَا فُرِضَتْ اَلصَّلَاةُ رَكْعَتَيْنِ، فَأُقِرَّتْ صَلَاةُ اَلسَّفَرِ وَأُتِمَّتْ صَلَاةُ اَلْحَضَرِ»
When the Salat (prayer) was first prescribed, it consisted of two Rakat. Afterwards, the prayer during travelling was confirmed (as two Rakat), while the prayer at the place of residence was completed (as four Rakat).
صحيح. رواه البخاري (1090)، ومسلم (685)
Who passed it on, from Ibn Hajar back to the one who was there.
The same words, in another collection.
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