Sayyiduna Anas bin Malik (may Allah be pleased with him) narrates: When the eyesight of Sayyiduna Itban bin Malik (may Allah be pleased with him) was afflicted, he sent word to the Messenger of Allah (peace and blessings be upon him): I would like you (peace and blessings be upon him) to come to me and pray in my house, and supplicate for blessing for us. So the Messenger of Allah (peace and blessings be upon him) set out with some of his Companions and went to his house. The people began talking among themselves and mentioned Malik bin al-Dukhshum. One of them said: O Messenger of Allah, he is the refuge and shelter of the hypocrites, and they said much about him. The Messenger of Allah (peace and blessings be upon him) said: "Does he not pray?" They said: Yes, O Messenger of Allah, but a prayer in which there is no good. The Messenger of Allah (peace and blessings be upon him) said: "I have been forbidden from killing those who pray." He (peace and blessings be upon him) said this twice.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 15135