Question: 

What is the travel distance that gives the concession to shorten the prayer? 

Answer:

In the name of Allah, we praise Him, seek His help and ask for His forgiveness. Whoever Allah guides none can misguide, and whoever He allows to fall astray, none can guide them aright.

Shortening of prayers is a recommended sunnah of the Prophet (peace be upon him). It is mentioned both in the Quran and is a confirmed sunnah. Almighty Allah says:

And when you travel throughout the land, there is no blame upon you for shortening the prayer, if you fear that those who disbelieve may disrupt [or attack] you. Indeed, the disbelievers are ever to you a clear enemy. (An-Nisa’ 4:101)

`A’ishah (may Allah be pleased with her) narrated that when the Salah (prayer) was first prescribed, it consisted of two rak`ahs. Afterwards, the prayer during travelling was confirmed (as two rak`ahs), while the prayer at the place of residence was completed (as four rak`ahs).” (Agreed upon)

There is no mention of the Prophet (peace be upon him) that he ever prayed in full while travelling. 

Watch this interesting talk with Sr. Muhammad Salah to learn more about the travel distance for shortening prayers.

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Source: Huda Youtube Channel.