Musannaf Ibn Abi Shayba 38:130

Waki’ narrated to us, from ‘Ikrimah ibn ‘Ammar, from ‘Asim ibn Shumaykh, who said: I heard Abu Sa’id al-Khudri saying, while his hands were like this—meaning they were trembling from old age—: "Fighting the Khawarij is more beloved to me than fighting the same number of them from the people of polytheism."