Musannaf Ibn Abi Shayba 18:345

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Ibn Abi Layla narrated to us, from ‘Ikrimah ibn Khalid, from a man from the family of ‘Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said regarding the man: "Fifty."