Musnad Ahmad 937:196

'Affan narrated to us, Hammad ibn Salamah narrated to us, Abu Na'amah informed us, from al-Ahnaf ibn Qays, who said: I arrived in Medina intending to receive the stipend from 'Uthman ibn 'Affan. I sat with a circle of the circles of Quraysh, and a man came wearing tattered clothes, having wrapped a garment over his head. He said: "Give glad tidings to the hoarders of a branding on their foreheads, and a branding on their backs, and a branding on their sides." Then he moved aside to a pillar and prayed two rak'ahs behind it. I said: "Who is this?" It was said: "This is Abu Dharr." I said to him: "What is this thing I heard you calling out?" He said: "I did not say to them anything except what they heard from their Prophet, may Allah bless him and grant him peace." I said to him: "May Allah have mercy on you, I used to take the stipend from 'Umar, so what do you see?" He said: "Take it, for it is today an assistance, and it is soon to become a debt; so when it becomes a debt, then abandon it."