Musnad Ahmad 27:1617

Ibrahim ibn Abi al-Abbas narrated to us, Abd al-Rahman ibn Salih ibn Muhammad al-Ansari narrated to me, from Umar ibn Abdullah, the freed slave of Ghufrah, from Nafi’, from Ibn Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Indeed, for every nation there are Magians, and the Magians of my nation are those who deny the Divine Decree (al-Qadar). So if they die, do not attend their funerals, and if they fall ill, do not visit them."