All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
Actually, Thuhr is the first prayer and thus it was called the first. Abu Barzah Al-Aslami said: “He, sallallaahu ‘alayhi wa sallam, used to pray the Thuhr prayer, which you call the first prayer, as the sun declined at noon…” [Al-Bukhari, An-Nisaa’i and Ahmad]
Ibn Hajar said in Fat-h Al-Baari: “It was called the first because it is the first prayer in the daytime and it was said because it is the first prayer in which Jibreel (i.e. Gabriel) led the Prophet, sallallaahu ‘alayhi wa sallam, when he explained to him the times of the five prayers (227).”Allaah Knows best.