Should the Fajr prayer be repeated if it was interrupted to catch the opening Takbir with the Imam?

If the prayer is called and you have entered into the intention for the Sunnah (definition) of Fajr, the correct course of action is to interrupt your prayer and prepare for the obligatory prayer. This is based on the saying of the Prophet, peace and blessings be upon him: "When the prayer is called, there is no prayer except the prescribed (obligatory) one." This Hadith (definition) supersedes the general verse: "And do not invalidate your deeds." However, if the prayer is called and you have already performed the second bowing (ruku'), then there is no harm in completing it. The regular Sunnah prayer of Fajr, which you interrupted, should be made up either immediately after the obligatory prayer or after the sun has risen.