about whom he is uncertain; or if he enters a prayer that must be performed complete, it is corrupted and he repeats it; or he failed to intend shortening when he began, or is unsure of that intention, or he delays the prayer until only a narrow span of its time remains (1); or he determines to reside for more than the span of twenty prayers—then he must perform the prayer in full.

But if he has two routes and chooses the longer one; or he remembers a travel-prayer during another journey; or he is kept back by rain or the like without intending to settle; or he stays to take care of some need without forming the intention of residence—then he may shorten.

Section
[Combining Two Prayers]
It is permissible to combine Zuhr with ‘Asr and Maghrib with ‘Isha’ in the time of either one (2) when one is on a journey that permits shortening, and for a sick person whose hardship would be aggravated by praying them separately. Maghrib and ‘Isha’ may likewise be combined because of rain that soaks the clothes and entails difficulty, or because of mud or a strong, cold wind, even when one prays at home.

The preferable course is to choose whichever of advancing or delaying is easier; if they are equal, delaying is better, and the two prayers must be kept in their proper order.

When the combination is made by advancing, the following conditions apply: the intention to combine must be formed at the opening takbīr of the first prayer; the two prayers may not be separated by more than the time needed for the iqāmah and a brief ablution; the combination is invalidated by performing a regular sunnah prayer between them; the legitimate excuse must exist at the commencement of both prayers and at the salutation of the first, and continue until the second is completed.

When the combination is made by delaying, it is required that the intention to combine be formed during the time of the first prayer before it becomes too constricted to perform, and that the excuse persist until the time of the second prayer begins.