When a person is praying the [the two units of] Sunnah prayers before fajr or [any prayer] other than fajr, and while doing so the iqaamah for the prayer is announced while he has still not yet finished praying his sunnahs, should he break his prayer in order to pray with the Imaam or should he complete his Sunnah [prayers first]?
Al-Albaani: The principle in this issue is the Prophet’s saying (صلى الله عليه وسلم), “When the iqaamh for prayer is called, there is no prayer except the prescribed prayer.” [Muslim, no. 1644].
So from this hadith it is clear that the [sunnah] prayer is nullified just by the fact that the prescribed prayer has been established–but the scholars differed as to whether the hadith is general without exception and whether it can be understood that a person can continue [to pray] in some circumstances to complete his sunnah prayer and then join the congregational prayer.
What appears to me [to be the answer] based upon what I have read from Imaam An-Nawawee’s book [called] Al-Majmoo’, is that what is intended by this hadith is to encourage the Muslim who prays the optional prayers to catch the [prescribed] prayer with the Imaam from the very start, i.e., that he should not miss the first saying of Allaahu Akbar which begins the prayer [takbeeratul-Ihraam].
So if the iqaamah for the [prescribed] prayer is called and the person is praying the optional prayer in the final sitting before giving salaam [i.e., tashahhud], and all that remains [for him to complete his optional prayer] is to give salaam, and that he thinks it is more than likely that he will be able to catch the first takbeer–then in this case he can complete his prayer, and finish it even if only with the first salaam [to the right] at the very least.
In contrast, if he said the takbeer to start praying his sunnah for fajr for example, and at the same time the iqaamah for the prescribed prayer is called such that if he were to continue until he completed his optional prayer it would mean that, at the very least, he would miss the first takbeer which begins the prayer with the Imaam [takbeeratul-Ihraam], then in this situation he should break his prayer, i.e., without giving salaam. And between these two situations are many others.
In summary, the person who is praying optional prayers when the iqaamah is called should make an effort to consider whether or not by continuing the optional prayer he will catch the initial, first takbeer which starts the prescribed prayer.
If he thinks it more than likely that he will catch it, he complete his prayer and keeps it short, and if he thinks it more likely that he will miss the first takbeer that commences the [prescribed] prayer, he breaks off his optional prayer and joins the row.
Al-Masaa’il al-‘Ilmiyyah wal-Fataawaa ash-Shar’iyyah, Fataawaa ash-Shaikh al-Allaamah Muhammad Naasirud-Deen al-Albaani, fil-Madeenah wal-Imaaraat, of Amr Abdul-Mun’im Saleem, pp. 100-101.