Sayyiduna Wahshi bin Harb (may Allah be pleased with him) said: I came to the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) said to me: "Wahshi?" I said: Yes. He (peace and blessings be upon him) said: "Did you kill Hamzah?" I said: Yes, and all praise is for Allah who honoured him with martyrdom at my hand and did not disgrace me at his hands. At that, Quraysh said to him (peace and blessings be upon him): Kill him, he is the killer of Hamzah. I said: O Messenger of Allah! Pray for forgiveness for me. So he (peace and blessings be upon him) spat on the ground three times and struck my chest three times, and said: "O Wahshi! Go out and fight in the path of Allah just as you fought to obstruct the path of Allah."
Hadith Referenceالمعجم الكبير للطبراني / باب الواو / 18590