Musnad Ahmad 692:1

'Abd al-Rahman narrated to us, from Sufyan, from 'Ata'—meaning Ibn al-Sa'ib—from a man from Bakr ibn Wa'il, from his maternal uncle, who said: I said: "O Messenger of Allah, should I collect the tithe (*'ushr*) from my people?" He said: "The tithes are only upon the Jews and the Christians, and there are no tithes upon Islam."