Musnad Ahmad 2:244

Abu Sa’id narrated to us, Dujayn Abu al-Ghusn, a Basran, narrated to us, he said: I arrived in Medina and met Aslam, the freed slave of Umar ibn al-Khattab, and I said: "Narrate to me about Umar." He said: "I cannot; I fear that I might add or subtract. When we used to say to Umar, 'Narrate to us about the Messenger of Allah, peace and blessings of Allah be upon him,' he would say: 'I fear that I might add a letter or subtract one. Indeed, the Messenger of Allah, peace and blessings of Allah be upon him, said: "Whoever lies about me, then he is in the Fire."'"