
Hazrat Abdullah bin Abu Qatadah reported from his father Abu Qatadah that the Prophet (peace be upon him) used to make the first rakaah of the Zuhr prayer longer than the second. He also did the same in the Fajr prayer. (Bukhari: 779)
Based on this hadith, Islamic scholars and jurists say that it is mustahab (recommended) to make the recitation in the first rakaah longer than in the second rakaah.