Jarīr ibn ‘Abdullāh (may Allah be pleased with him) reported that the Messenger (may Allah's peace and blessings be upon him) said: "He who does not show mercy to people, Allah will not show mercy to him."
Explanation:
He who does show mercy to people, Allah, Exalted and Glorified, will not show mercy to him. The word 'people' refers to those who deserve mercy, such as Muslims, non-Muslims under Muslims' protection, and the like. As for combatant non-Muslims, they aren't to be shown mercy. They should be killed. Allah described the Messenger and his companions in the Qur'an: {severe against disbelievers, but merciful among themselves.} [Sūrat al-Fat'h: 29]