Is the adhan (call to prayer) an announcement of the beginning of the time for prayer, or for the prayer itself?

Linguistically, the adhan means 'proclamation'. Legally (in Sharia), it is a proclamation of the arrival of the prayer time and an invitation to congregational prayer. It also serves to manifest the rituals of Islam. This is supported by what the two Sheikhs (Bukhari and Muslim) narrated from Ibn Umar: "When the Muslims arrived in Medina, they would gather and wait for the prayers. Some suggested a bell or a horn. Umar said: 'Why don't you send a man to call for prayer?' Thereupon, the Messenger of Allah (peace and blessings be upon him) said: 'O Bilal, stand up and call for prayer.'" And what the two Sheikhs narrated from Anas: "When the Prophet (peace and blessings be upon him) raided a people, he would not raid until morning. If he heard an adhan, he would refrain from them. If he did not hear an adhan, he would launch an attack on them."