Is it permissible to offer the two Sunnah rakat at the Fajr while congregation prayer of the fardh is going on? Please give some clarification and references.
All perfect praise be to Allah
First i should tell that some scholars moslty Hanafi are on the view that it is permissible to do so, and they even encourage it.
And they quote some references like:
Ibn Masood(ra) came and the Imam was in the Fajr prayer. He performed the sunnah prayer behind a pillar and then joint the Imam. (I’la al-sunan 7/101)
The main reason is that they say that sunnah of fajr cannot be prayed after farz prayer.
But the right view is that these sunnah can be performed after farz
"The Prophet (Sallallahu Alaihi wa Sallam) saw me performing two Rak'ah of prayers after finishing the Fajr prayer He asked: 'What are these two Raka'h?' "What is this prayer?" I said: 'O Messenger of Allah! I did not perform the two Raka'h that precede the prayer of Fajr, so I performed them now" [ Ahmad , Abu Dawood and al-Tirmizi ].
The Prophet (Sallallahu Alaihi wa Sallam) did not make any comment. So, the silence of the Prophet (Sallallahu Alaihi wa Sallam) proves that it is permitted.
For details see here
Now, the correct view about the question being asked is that it is not right to offer sunnah or nafal prayer after iqqamah.
Abu Hurayrah may Allaah be pleased with him narrated that the Prophet , said: “When the Iqaamah for prayer is said, there is no prayer except the prescribed prayer.” [Muslim and others]
"The Iqaamah for the Fajr prayer was being called and the Prophet sallallaahu alayhi wa sallam ( may Allaah exalt his mention ) saw a man praying while the person who calls the Athaan was calling the Iqaamah, so he sallallaahu alayhi wa sallam ( may Allaah exalt his mention ) told him: ''Are you performing the Fajr prayer four Rak'ahs?" [Muslim]
Now, these two are from Sahi muslim, which means these hadees are sahi. And it clearly states that there is no other prayer after iqaamah, except prescribed prayer.
Also Bukhari entitled a chapter in his Sahih with the title: "Only compulsory prayer after Iqama"
So, these evidence from SAHI muslim are more reliable and have direct saying and doing of prophet muhammad (sallallaahu alayhi wa sallam) rather than the less authentic ahadees presented like the first one from I’la al-sunan and which tells practice of some sahaba, not direct orders and actions of prophet.
So, at-least i will go with more authentic ahadees, and direct orders of Prophet Muhammad(PBUH) rather than going with less authentic ones.