Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.118.

Ahmad, Qadi Hussain, al-Khattabi, and al-Mutawali of the Shaf’i school are of the opinion that it is allowed to combine two prayers, either during the time of the earlier or later salah, due to illness as it is a greater hardship than rain. An-Nawawi says: “This is a strong opinion based on [sound] evidence.” In al-Mughni it is stated: “The illness which permits one to combine the prayers is the one which would otherwise cause hardship and more weakness [if he prayed each salah separately].”

The Hanbali school is the most accommodating as it allows one to combine the prayers, at the time of the early or later salah, for one who is ill as well as for the woman who is breast-feeding and will face hardship in cleaning her dress for every salah, for the woman who is plagued by a prolonged flow of blood, for the person who cannot control his urine, and for one who cannot purify himself or herself, and for the one who fears for his life, property, or family.

Ibn Taimiyyah says: “Among the opinions the most accommodating on this question is that of the Hanbali school which allows one to combine the prayers if he is busy (since an-Nasa’i has related something to that effect from the Prophet sallallahu alehi wasallam) and they also say that the cook and baker, and so forth., may also combine their prayers if they fear their wealth (i.e., their investment or what they are working on) will otherwise be ruined.”