Musnad Ahmad 1034:149

Waki’ narrated to us, from Sufyan, from ‘Ata’ ibn al-Sa’ib, from Abu al-Bakhtari, who said: Hudhayfah said: "The companions of the Prophet, may Allah bless him and grant him peace, used to ask him about the good, and I used to ask him about the evil." It was said: "Why did you do that?" He said: "Whoever guards against evil falls into good."