Shaykh ‘Abdur-Rahmaan ibn Naasir as-Sa’dee (d.1376), may Allaah have Mercy on him, was asked about the proper application of the authentic hadeeth that means,
“Once the prayer is established there is no prayer other than the obligatory one.”
Recorded by Muslim, Abu Dawud, Tirmizi, Ibn Majah and Nasa’i
The scholars have mentioned that it is applied to initiating a new optional prayer (that has not been started yet) by someone intending to pray with the imaam, that it is not allowed in this case. However, they excluded someone finishing a prayer (already started before theiqaamah was called) from this ruling, to combine between it (this hadeeth) and the Statement of Allaah:
ولا تبطلوا أعمالكم
“And do not render your deeds invalid.”
There is a difference between initiating (a new prayer) and finishing (a prayer already started), the latter having an easier ruling.
Know that there are four or five situations related to this issue (involving people when they hear the iqaamah):
1) If the iqaamah is called while someone has not started praying an optional prayer, he must not pray an optional prayer in this case. This is the clearest violation of the hadeeth.
2) If the iqaamah is called while someone has already initiated an optional prayer, and finishing the optional prayer would cause him to miss the congregational prayer, or arak’ah of it according to the better of the two positions of the scholars, such a person isabsolutely required to discontinue his optional prayer, since there is no (acceptable) clash between an obligation and a recommendation. And the generality of the obligation of the congregational prayer includes this scenario as well. Having started an optional prayer does not excuse a person from the obligation (of joining the congregational prayer).
3) If (the iqaamah is called while) someone is praying, and he can finish his prayer (all the way) to the salaams, it is best that he finishes the prayer. This is the clearest scenario that is referred to in the words of our fellow (Hanbalee) scholars who said, “If he has already begun it, he finishes it concisely.”
4) If (the iqaamah is called while) someone is praying, and he is not sure if he can finish it in time to catch the first rak’ah or not, the generality of the statements of the (Hanbalee) scholars indicates that it is better to complete it concisely, even if he actually misses therak’ah. Some others in the (Hanbalee) school of Fiqh have said that it is better to discontinue in this case, and this is what I believe to be correct, due to the generality of the hadeeth and the permissibility of discontinuing an optional deed. Furthermore, obligations and their benefits are not comparable to optional deeds. A single obligation is better than many optional deeds.
And If this is the case regarding just one rak’ah (of obligatory prayer being missed), then missing more would be even more important to avoid.
Source: Al-Fataawee as-Sa’diyyah (pp.168-169)
Translated by: Moosaa Richardson