Hazrat Ibn Umar (may Allah be pleased with them both) used to say that if you remember during the Asr prayer that you have not performed the Zuhr prayer, then continue performing the Asr prayer, and after that perform the Zuhr prayer. If you remember after performing the Asr prayer that you have not performed the Zuhr prayer, then your Asr prayer is valid.

It is also narrated in the same way from Hasan.

Ibrahim, regarding the person who leads people in Zuhr prayer while it is actually his own Asr prayer, says that his prayer will be valid, but the followers will have to repeat their prayer.

Hazrat Abu Qalabah says that one congregation cannot be for two different prayers.

Hazrat Abad bin Mansoor says that I went to the central mosque and saw that the people had not yet performed the Zuhr prayer. While waiting for Zuhr, I started offering nafl (voluntary) prayers. In the meantime, the congregation stood up. When they finished the prayer, I found out that it was the Asr prayer. So I then performed my Zuhr prayer. Then I performed the Asr prayer. Then I went to Hasan and asked him about this issue, and he gave me the same ruling as what I had done.

Hazrat Kathir bin Aflah says that I reached the mosque, and I had not yet performed the Maghrib prayer. In the meantime, the congregation stood up, and I joined them in prayer, intending it as Maghrib. But I found out that they were actually performing the Isha prayer. So I first performed the Maghrib prayer and then the Isha prayer. Then I asked the elders about this matter, and they instructed me to do exactly what I had done.

Hazrat Zuhri, regarding the person who joins a congregation thinking it is the Zuhr prayer, but those people are actually offering the Asr prayer, states that he should first offer the prayer which Allah has prescribed first, that is, Zuhr, and then offer the Asr prayer.

Hazrat Tawus and Hazrat Ata say that in this way too, his prayer will be valid.

Hazrat Jabir says that I asked Hazrat Abu Ja'far, Hazrat Salim, Hazrat Qasim, and Hazrat Ata about a person who joins a congregation thinking it is the Zuhr prayer, but they are actually performing the Asr prayer. They replied that after completing the prayer with salam, he should perform the Zuhr prayer; however, his Asr prayer will be valid. He says that I asked Hazrat Amir and Hazrat Muslim bin Subaih about this, and they replied that after finishing, he should first perform the Zuhr prayer and then the Asr prayer, because Allah the Exalted has made Zuhr obligatory before Asr. Therefore, the Zuhr prayer cannot be performed after Asr. Hazrat Jabir states that this is also the opinion of Hazrat Hammad and Ibrahim.

4850 - And Jabir narrated from Hammad from Ibrahim the like of that.