When was first Salat with congregation was obligatory? In Makkah or Madina? In which Hijrah year? In time of Rasoolillah (Sallallahu Alaihi wa Sallam) was any congregational prayer conducted without Azan and when? Is the Azan compulsory for congregational Salat?

All perfect praise be to Allah, The Lord of the Worlds. I testify that there is none worthy of worship except Allah, and that Muhammad is His slave and Messenger.

The prayers were made obligatory at the night of the Prophet’s Israa’ (nocturnal journey) and al-Mi'raaj (ascension) to the seventh heaven; they were revealed to him directly at the 7th heaven. But there is a disagreement among scholars concerning the year, month and day of al-Israa’ and al-Mi'raaj.

As for performing the prayers in congregation, it became a manifest rite just after the Prophet built his mosque in the first year of Hijrah.

But is it obligatory to perform it in congregation? Scholars of Islam differ concerning such an issue. Some scholars believe that it is obligatory for those obliged to perform the Jumu'ah prayer.

Others believe that it is a collective duty; i.e. if some people perform it in congregation, the others are acquitted from sin. A third group believe that it is just a Sunnah to perform in congregation. Any way, each group have their proofs and evidence. But we think that the first opinion is the nearest to what is right.

As for your question about the Aathaan, we say that the Aathaan should be done in the mosque; i.e. it is obligatory according to the preponderant opinion of the jurists, but it is not a condition for the validity of the prayer.

Allah knows best.