Riyad Us-Saliheen (Gardens of the Righteous)

Chapter 344
Undesirability of offering optional Prayer after the announcement of Iqamah

1759. Abu Hurairah (May Allah be pleased with him) said: The Prophet (PBUH) said, "When the Iqamah is called, no prayer should be performed except the obligatory prayer.''
[Muslim].

Commentary:  When the Iqamah has been announced (prescribed recitation at the commencement of Salat when offered in congregation, after the worshippers have taken up that position), it is not permissible to offer any other prayer - whether it is Sunnah or Nafl. If a person has already started any, he should break it to join the congregation. He should offer the Salat which he terminated after the obligatory Salat offered in congregation. It is against this Hadith to continue Nafl Salat or Sunnah when the worshippers have taken the position to offer Salat in congregation and Iqamah has been called. Some scholars are of the opinion that one can continue the performance of Fajr Sunnah, even after the congregational Salat has started. They have taken the plea that if there is no risk of losing the first Rak`ah, it is permissible to continue the Sunnah of Fajr prayer. Their plea is against the present Hadith. It is a common feature to see the worshippers offering Sunnah during the course of Fajr prayer offered in congregation. Thus in practice, the Sunnah of the Fajr prayer are continued regardless of this stipulation. In any case, this practice is totally against the injunctions contained in this Hadith.