His statement: “Whoever begins an obligatory act whose time is extended is forbidden to discontinue it.”
Obligations fall into three categories: those whose time is extended; those whose time is restricted by their very legislation; and those whose time becomes restricted due to a subsequent circumstance. An example of a subsequently restricted time is when only enough time remains before sunrise to perform the Fajr prayer. In that case, the time has become restricted, and once one begins the Fajr prayer, it is not permissible to discontinue it.
Likewise, making up missed fasts of Ramadan may ordinarily be done at any time; but when only as many days remain until Ramadan as the number of fasts one owes, the time becomes restricted.
“Whoever begins an obligatory act whose time is extended is forbidden to discontinue it. This is not required in the case of a voluntary act, nor is one required to make up a voluntary act that has been invalidated …”
His statement, “Whoever begins an obligatory act whose time is extended is forbidden to discontinue it,” means that whoever commences an obligation whose time is extended is forbidden to stop it, and must complete it unless there is a valid legal excuse.
For example, when the call to the Zuhr prayer is given, a person stands and performs the Zuhr prayer, then wishes to stop the prayer and perform it later. This is not lawful for him, even though its time extends until Asr, because it is an obligation that he has begun; beginning it is akin to making a vow, so he must complete it.
Whoever begins an obligation whose time is restricted is, all the more, forbidden to discontinue it. Thus, if he begins the prayer when only enough time remains for its required units, it is even more prohibited for him to stop it. For if discontinuing it is forbidden when the time is extended, then it is even more so when the time is restricted.
An exception is made in cases of necessity: for example, when a person begins the prayer and then must interrupt it to extinguish a fire, rescue someone from drowning, or the like. In such a case, he may interrupt the prayer.
But may one interrupt an obligatory act in order to perform it in a more complete manner—for example, if he