Musnad Ahmad 457:13

Ali ibn Ishaq narrated to us, Abdullah—who is Ibn al-Mubarak—narrated to us, he said: Al-Awza'i narrated to us, from Yahya ibn Abi Kathir, from Abu Qilabah, from Abu Mas'ud al-Ansari, who said: It was said to him, "What did you hear the Messenger of Allah, peace and blessings of Allah be upon him, say regarding 'they claim' (za'amu)?" He said: "It is the worst mount for a man."