From Sahih Bukhari (Hadith 1531):
Narrated Ibn `Umar:When these two towns (Basra and Kufa) were captured, the people went to `Umar and said, "O the Chief of the faithful believers! The Prophet (ﷺ) fixed Qarn as the Miqat for the people of Najd, it is beyond our way and it is difficult for us to pass through it." He said, "Take as your Miqat a place situated opposite to Qarn on your usual way. So, he fixed Dhatu-Irq (as their Miqat)
Reference: Bukhari 1531