Musannaf Ibn Abi Shayba 28:876

Waki’ narrated to us, he said: Abu Usamah narrated to us, from Zayd, from Aban ibn Salih, from Mujahid, who said: Umar said: "Any man from the Muslims who gestures to a man from the enemy so that he may descend [from his position] in order to kill him, and he descends while he [the enemy] believes that it is a guarantee of safety, then he has indeed granted him safety."