Mishkat al-Masabih 30 Fine Qualities كتاب المناقب 67

وَعَنْ عُقْبَةَ بْنِ عَامِرٍ قَالَ: قَالَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «لوكان بَعْدِي نَبِيٌّ لَكَانَ عُمَرَ بْنَ الْخَطَّابِ» . رَوَاهُ التِّرْمِذِيُّ. وَقَالَ: هَذَا حَدِيثٌ غَرِيبٌ

`Uqba b. `Amir reported the Prophet as saying, “If there were to be a prophet after me, he would be `Umar b. al-Khattab.” Tirmidhi transmitted it, saying this is a gharib tradition.