Hazrat Shaikh Moulana Muhammad Zakariyya (rahmatullahi ‘alaih) once mentioned the following:
There is no need for a person to impose undue hardship on himself with regards to fulfilling his daily zikr. Hence, if due to sickness, one finds it difficult to complete his daily zikr, then there is no harm.
As far as one’s recitation of the Qur’aan Majeed is concerned, then greater importance should be shown to completing one’s recitation. If a person is not so sick, then he should make an effort to complete his recitation, even though he recites softly.
Regarding one’s zikr and other ma’moolaat (prescribed daily practices), then one should try not to leave it out entirely. Rather, at the time of difficulty or illness, one should at least complete some portion of the zikr and ma’moolaat.
If one falls ill, then one should not feel that his illness is on account of some evil action which he had done. Rather, sickness is something which is decreed from Allah Ta‘ala. Therefore, instead of one pondering over the sickness, one should rather be concerned that his zikr and ma’moolaat are not left out entirely. Nevertheless, if due to illness, one misses his ma’mool now and again, then there is no problem with this.
(Qutbul Aqtaab Hazrat Sheikhul Hadith Moulana Muhammad Zakariyya (rahmatullahi ‘alaih) pg. 401-402)