Musannaf Ibn Abi Shayba 28:877

Waki’ narrated to us, he said: Musa ibn Ubaydah narrated to us, from Talhah ibn Ubayd Allah ibn Kurayz, he said: Umar wrote to the commanders of the armies: "Whichever man among the Muslims gestures to a man from the enemy, saying, 'If you descend, I will surely kill you,' and he descends while he (the enemy) believes that it is a guarantee of safety, then he has indeed granted him safety."