It is also understood from the author’s words that if the movement is slight, the prayer is not invalidated. But what is the criterion for a movement’s being slight or extensive?

The answer is that the author indicates that the criterion is customary practice. In truth, however, customary practice is somewhat ambiguous and can hardly be precisely defined, since customs differ from one land to another and according to people’s perceptions. Some people may regard a matter as extensive, while others may regard it as slight. The closest approximation is to say: if we see a person moving, and it becomes most likely in our judgment—because of the amount of his movement—that he is not praying, then this should be the criterion: the movement should be such that whoever sees the person doing it assumes that he is not in prayer, for this is what is incompatible with the prayer.

As for that which is not incompatible with the prayer, but is merely slight movement, it does not invalidate the prayer.

Some scholars have defined excessive movement as three movements (1), but this definition is not sound. The Messenger of Allah, peace and blessings be upon him, opened the door for Aishah while the door was in the direction of the qiblah; he stepped forward and then returned (2). During the eclipse prayer, he stepped forward, stepped back, and moved further back (3). And when a pulpit was made for him, he began praying upon it: he would ascend it while standing and bowing, then descend to the ground for prostration (4).