to both the manner of the prayer—its form—and its number. Just as the Messenger of Allah, may Allah’s peace and blessings be upon him, never prayed more than two units while travelling, and we have been commanded to pray as he prayed.
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It was the Prophet’s constant practice.
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This was also narrated from Ibn Abbas, Ibn Umar, and others. (1)
However, a number of principles weigh against the view that shortening is obligatory:
The first principle is that a traveller praying behind a resident prays four units in following the imam. Following the imam is obligatory, while adding to the prescribed prayer invalidates it. Therefore, if your imam rises for a fifth unit and you are certain that it is the fifth, you must cease following him and must not continue with him. Thus, we say: if shortening were obligatory, then following the imam in completing the prayer would be unlawful—just as when a person prays the dawn prayer behind someone praying the noon prayer: he cannot follow him for all four units; rather, when the imam rises for the third, he remains seated.
However, this principle may be countered by saying: adding beyond four units is impermissible only when the imam rises for a fifth, because such a prayer has not been legislated—that is, no prayer consisting of five units has been prescribed. As for a traveller following an imam who performs the prayer in full, this is legislated; indeed, it is the original ruling for the prayer of a resident. Thus, there is a difference between the two cases. Likewise, we say regarding one who prays the dawn prayer behind someone praying the noon prayer: he cannot rise with him and complete four units, because the dawn prayer cannot consist of four units, whether one is resident or travelling—unlike one who follows the imam in a shortened prayer while the imam performs it in full.