Abu Hurairah reported,
The Messenger of Allah ﷺ said, "Whoever helps in the killing of a believer, even by half a word, will meet Allah 
with the words 'despairing of Allah's mercy' written between his eyes."
Ibn Majah 2620
Daifحدثنا عمرو بن رافع حدثنا مروان بن معاوية حدثنا يزيد بن زياد عن الزهري عن سعيد بن المسيب
عن أبي هريرة قال
قال رسول الله ﷺ من أعان على قتل مؤمن بشطر كلمة لقي الله عز وجل مكتوب بين عينيه آيس من رحمة الله
No references are available.
[Daif] Bayhaqi (8/22) narrated it from the hadith of Marwan bin Muawiya Fazari, with the same chain, and he said, Yazid bin Ziyad, also said to be Ibn Abu Ziyad Shami, is munkar al-hadith. Abu Hatim said this hadith is baseless, and Busiri graded it daif. The hadith has daif corroborating narrations in Bayhaqi and Abu Naeem (Hilyah, 5/74), which is fabricated, and elsewhere.