The first view is that establishing the prayer means commencing it; that is, the opening takbir.
The second view is that establishing it means beginning the iqamah—the announcement that the people are to rise for the prayer.
The third view is that it means the completion of the iqamah. This view is close to the first, although the imam may sometimes be delayed in completing the iqamah, either while straightening the rows or because of some excuse that arises, or the like.
However, once we know the wisdom behind the prohibition, we can determine what is meant by establishing the prayer. The wisdom behind the prohibition is that a person should not occupy himself with a voluntary prayer that he performs alone alongside an obligatory prayer being performed in congregation, for in that case he differs from the people in two respects:
First, he is engaged in a voluntary prayer while the people are engaged in an obligatory prayer.
Second, he is praying alone while the people are praying in congregation.
It is well known that if a person begins a voluntary prayer after the one calling the iqamah has begun it, he will generally not finish it until the people have commenced the congregational prayer. Accordingly, it is not permissible for him to begin a voluntary prayer after the caller has begun the iqamah, because the reason for the prohibition exists in this situation. All the more so, he should not begin the voluntary prayer once the iqamah has ended or once the imam has commenced the prayer.
Accordingly, the Prophet’s statement, peace and blessings be upon him, “There is no prayer except the prescribed prayer” (1), means: