Musnad Abu Ya'la al-Mawsili 155:60

Yahya ibn Ayyub narrated to us, Marwan ibn Mu'awiyah narrated to us, Yazid ibn Ziyad al-Shami informed us, from al-Zuhri, from Sa'id ibn al-Musayyib, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Whoever assists in the killing of a Muslim with even half a word, he will meet Allah on the day he meets Him with 'Despairing of the mercy of Allah' written upon his forehead."