In the discussion about offering the two rakah Sunna salah prior to Fajr once iqamah has been called, sayyidi wa sanadi Mufti Mohammad Taqi Usmani (Allah preserve him) said,
‘The authentic saying of Imam Abu Hanifa (Allah have mercy on him) and dhahir al rewaya is also that with two criteria being fulfilled it is permissible to offer this salah. Firstly, it is expected that one complete rakah (of the fardh salat al fajr) will be attainable. Secondly, that they must be offered out side the mosque.
Imam Mohammad (Allah have mercy on him) said that even if not one complete rakah only the last sitting (qaida al akhira) is expected to be attainable than also it is permissible to offer these Sunna .
Imam Mohammad (Allah have mercy on him) added further to the first criteria but not to the second one (that is to offer it outside the mosque).
Imam Tahawi (Allah have mercy on him) added to the second criteria. He said offering them outside mosque it is permissible, however, inside the mosque is also permissible provided it is far away from the rows of the congregational salah and in an isolated spot.
From all this discussion it is clear that the prevalent practice of offering this salah immediately behind the congregation or few rows away is not permissible according to any of the madhahib.
The predominant (viewpoint) is to follow the original way (madhab) of Imam Abu Hanifa (Allah have mercy on him). The Hanafi researches have given preference to this. However, Allama Shami (Ibne Abideen) keeping in consideration the feeble Muslims has given fatwa according to Imam Tahawi (Allah have mercy on them).
Whereas, our Hanafi researchers, that include respected (Shaikh Anwar) Shah (Kashmiri, Allah have mercy on him) do not agree. They stick to the original madhab of Imam Abu Hanifa (Allah have mercy on him). That is, one rakat is expected to be attainable and it is offered outside the mosque.
Another issue to be acknowledged is that the permissibility Imam Tahawi (Allah have mercy on him) gave for offering this salah in mosque was during an era when loud speaker were nonexistent and there was no clash between the recitation of the Imam and one offering his salah in an isolated corner. Nowadays when the recitation in (congregational) salah is being relayed on the loud speaker I have doubts about the permissibility of offering this (Sunna) salah in mosque. This is because there is clash of recitation in this situations. It is, therefore, best to offer it outside the mosque.’
Im’anul Bari, volume 3, page 416-7