Musnad Ahmad 26:435

Asbat narrated to us, Ash'ath narrated to us, from Kardus, from Ibn Mas'ud, who said: "The notables of Quraysh passed by the Messenger of Allah, may Allah bless him and grant him peace, while Khabbab, Suhayb, Bilal, and Ammar were with him. They said: 'O Muhammad, are you satisfied with these?' Then the Quran was revealed concerning them: 'And warn with it those who fear that they will be gathered to their Lord' [Al-An'am: 51] — up to His saying — 'And Allah is most knowing of the wrongdoers' [Al-An'am: 58]."