so be keen to pray two rak‘ahs while travelling, and do not add to them. However, if the imam completes the prayer, then you must complete it with him, lest you fall into opposition. This is in view of the Sacred Law’s concern for the unity of the ummah, even though it is contrary to what is preferable for you had you prayed on your own.
His statement, “It is prescribed for him to shorten a four-rak‘ah prayer”: the two-rak‘ah and three-rak‘ah prayers are excluded by the qualification “four-rak‘ah,” and so they are not shortened, because nothing to this effect has been reported from the Prophet, peace and blessings be upon him. We have an important principle: just as doing an act is sunnah, refraining from it when there is a reason to do it is also sunnah—even though this is an omission rather than an act.
One example is the recommendation of using the tooth-stick upon entering the mosque. Some scholars said that it is recommended for a person to use the tooth-stick when entering the mosque, basing this on the report that “when the Prophet, peace and blessings be upon him, entered his house, he would begin with the tooth-stick” (1). They analogized entering the mosque to entering the house and said: if a person uses the tooth-stick when entering his house so as to meet his family with a clean mouth, then likewise, when he enters the mosque to privately converse with his Lord, he should do so with a clean mouth.
We say, however: the Prophet, peace and blessings be upon him, used to enter the mosque, yet it has not been reported that, when he entered it, he began with the tooth-stick. Had this been sunnah, the Prophet, peace and blessings be upon him, would have done it. Thus, the sunnah is not to use the tooth-stick upon entering the mosque when the reason for using it is merely entering the mosque. But if, upon entering the mosque, he is going to pray two rak‘ahs immediately and wishes to use the tooth-stick for the prayer, rather than for entering the mosque, then this is legislated.
His statement, “when he has departed from the inhabited area of his town”: this is the condition for beginning to shorten the prayer; that is, he does not shorten it until he has departed from the inhabited area of his town.