Musnad Ahmad 27:1189

Abu Abd al-Rahman Abdullah ibn Yazid narrated to us, Sa’id—meaning Ibn Abi Ayyub—narrated to us, Abu Sakhr narrated to me, from Nafi’, who said: Ibn Umar had a friend from the people of Ash-Sham with whom he corresponded. Abdullah ibn Umar wrote to him once: "It has reached me that you have spoken about something regarding the Qadar (divine decree). So, do not write to me, for I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'There will be people in my Ummah who will deny the Qadar.'"