Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says regarding this statement of Allah Almighty, (Allah Almighty says:) "Mankind was one nation." Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says: It means that at first all the people were followers of Islam. Al-Kalbi says: It means that at first all the people were upon disbelief.
Abu Ya'la recorded it, and al-Tabarani recorded it in abridged form; the narrators of Abu Ya'la are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب التفسير / 10857