What is the ruling on one who habitually delays performing the two rak'ahs of Fajr (sunnah) until immediately after the Fajr (obligatory) prayer, in order to coincide with the iqamah of the prayer in the mosque?

Delaying the two rak'ahs of Fajr and making them up immediately after the obligatory prayer is not sinful. In fact, some scholars are of the view that they are considered performed on time (ada'), even though it is better to perform them before the obligatory prayer, due to the Hadith of 'Aisha (may Allah be pleased with her) in which the Prophet (peace and blessings of Allah be upon him) said: "The two rak'ahs of Fajr are better than the world and all that it contains." It is preferable to perform the Sunnah of Fajr at home, like the rest of the regular supererogatory prayers.