Musnad Abu Ya'la al-Mawsili 155:334

It was narrated to us by Muhammad ibn ‘Abbad al-Makki, who said it was narrated to us by Marwan, from Yazid, from Abu Hazim, from Abu Hurayrah, who said: It was said, "O Messenger of Allah, supplicate against the polytheists." He said, "Indeed, I was not sent as a curser; I was only sent as a mercy."