When should one go to congregational prayer: upon the Adhan (call to prayer) or upon the Iqamah (call to commence prayer)?

Congregational prayer in the mosque is considered Sunnah (definition) by the majority of scholars, and obligatory according to the Hanbalis. Based on the view of obligation, حضور to the mosque is required upon hearing the iqama. One must interrupt any supererogatory (nafila) prayer and join the obligatory prayer if the iqama has been called. Whoever catches the Imam before the taslim (definition) has caught the congregation. It is preferable to consistently catch the opening takbir (takbirat al-ihram (definition)). Attending before the adhan (definition) holds great virtue, and many of the early Muslims (Salaf) used to do so.