In one narration the words are: The Prophet (peace and blessings be upon him) said: "Why do you not turn your attention to Abd al-Rahman bin Awf? For he is the leader of the Muslims and one of the best of them."
Imam al-Tabarani recorded this narration in al-Mu'jam al-Awsat. The chain of the first narration contains a narrator, Ya'qub bin Humayd, and a narrator, Sulayman bin Salim; both of them have been declared trustworthy, and the remaining narrators of this narration are the narrators of the Sahih, while the second narration is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب المناقب / 14894