In the Hadeeth in which the Messenger of Allaah, sallallaahu ‘alayhi wa sallam (may Allaah exalt his mention), sent Mu‘aath Ibn Jabal, may Allaah be pleased with him, to Yemen to invite the people there to be Muslims, most of the pillars of Islam were mentioned [i.e. the two testimonies of faith, Salaah (prayer) and Zakaah], but excluding Sawm (fasting) and Hajj (pilgrimage). Why?

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, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

Al-Bukhari, At-Tirmithi, An-Nasaa’i, Abu-Daawood, Ibn Maajah and Ahmad narrated the Hadeeth of Mu‘aath . Sawm and Hajj were not mentioned in this Hadeeth because the Sharee‘ah pays more attention to Salah and Zakah. Therefore, Salah and Zakah were repeatedly mentioned.

Moreover, when it becomes obligatory for a Muslim to do Salah and Zakah, he is never exempted from doing them, unlike fasting and Hajj. Patients with chronic incurable diseases and the elderly who are unable to do Sawm are exempted from it. Instead, they should feed one needy person for each day they do not fast.

Also, a person can do Hajj on behalf of another person. For example, those who are unable to do Hajj due to suffering from a chronic disease or a severe illness or being unable to walk or to stay firmly on means of transportation can authorize another person to do Hajj on their behalf.

Allaah Knows best.