Ishaq bin Sa'id (may Allah have mercy on him) narrates from his father that al-Hajjaj ordered Salim bin Abdullah to kill a man. Salim asked the man: Did you pray the morning prayer? The man said: Yes. Salim said: Go away. Al-Hajjaj asked him: What stopped you from killing him? Salim said: My father told me that he heard the Messenger of Allah (peace and blessings be upon him) say: "Whoever prays the morning prayer remains under the protection of Allah for that day." So I disliked killing a man whom Allah had given protection. At this, al-Hajjaj asked Sayyiduna Ibn Umar (may Allah be pleased with him): Did you hear this from the Messenger of Allah (peace and blessings be upon him)? Sayyiduna Ibn Umar (may Allah be pleased with him) said: Yes.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 13154