All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger. We ask Allaah to exalt his mention as well as that of his family and all his companions.
The narration about this incident when the Prophet sent Mu’aath to Yemen is reported by Al-Bukhari, At-Tirmithi, An-Nasaa'ee, Abu Daawood, Ibn Maajah and Ahmad. The wisdom in this is that Islam concentrates on the prayer the prayer and Zakat, and it is for this reason that they were so often repeated in the Quran. When the prayer and Zakat become obligatory on the pubescent, he will never be exempted from them. On the contrary, a person who has a chronic disease, or an old man and an old woman who could not fast, could be exempted from fasting, however, they are required to feed a poor person for every day they break the fast.
Similarly, a person who is not able to perform Hajj because of his chronic disease or illness, or because of being unable to walk, or unable to get on a means of transport, and the like, is exampled and another person could perform Hajj on his behalf.
Allaah Knows best.