A man joined the Thuhr (noon) congregational prayer with the intention of performing the ‘Asr (afternoon) prayer. He remembered his mistake after the end of the first Rak‘ah (unit of prayer), what should he do? May Allaah Almighty reward you.

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.

If you mean that you joined the Thuhr congregational prayer while wanting to perform the ‘Asr prayer, but you intended Thuhr instead or did not make an intention at all; then your ‘Asr prayer is invalid in this case. The intention is a prerequisite for the validity of prayer, as the Prophet, sallallaahu ‘alayhi wa sallam, said: "Deeds are based on intentions, and each person will have but that which he intended." [Al-Bukhari and Muslim]

Additionally, the intention must be there at the beginning of the prayer or shortly before that. When one remembers during the prayer that he had the wrong intention or did not intend at all, he is obliged to break off his prayer and start over. If he continued praying or did not remember until after the end of the prayer, then he has to make up for the prayer.

If you mean that you were inattentive in prayer and thought you were praying Thuhr after intending ‘Asr at the beginning, then your prayer is valid in this case, as it is difficult to be attentive all the time. Moreover, this is not a prerequisite for the prayer to be valid.

Allaah Knows best.