Musnad Ahmad 1086:1

Hajjaj ibn Muhammad narrated to us, Layth narrated to us, Yazid ibn Abi Habib narrated to me, from Abu Ufayr al-Ansari, from Muhammad ibn Sahl ibn Abi Hathma, from Muhayyisa ibn Mas’ud al-Ansari: That he had a cupper slave called Nafi’ Abu Tayyiba. He went to the Messenger of Allah (peace and blessings of Allah be upon him) to ask him about his earnings (kharaj). He said: "Do not approach it." He repeated the request to the Messenger of Allah (peace and blessings of Allah be upon him), so he said: "Feed it to the camel used for watering, and put it into its stomach."