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?
Praise be to Allah, the Lord of the World; and may His blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.
The prayers were made obligatory at the night of the Prophet (Sallallahu Alaihi wa Sallam)'s Israa (nocturnal journey) and al-Mi'raaj (ascension) to the seven heavens; they were revealed to him directly at the 7th heaven. But, there is 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 (Sallallahu Alaihi wa Sallam) 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 al-Jumu'ah prayer.
Others believe it is a collective duty; i.e. if some perform it in congregation, the others will not have sinned. 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 Azan, we say that Azan should be done in a mosque, i.e. it is obligatory according to the dominant view among Fuqaha, but it is not a condition for the validity of prayer.
Allah knows best.
You can search for fatwa through many choices