Musannaf Ibn Abi Shayba 33:863

Husayn ibn Ali narrated to us, from al-Muhallab ibn Uqbah, who said: Umar ibn Abd al-Aziz used to deliver a sermon and say: "Indeed, among the most beloved of matters to Allah are moderation during prosperity, forgiveness when having the power to punish, and gentleness in authority. And no servant shows gentleness to another servant in the world, except that Allah will show gentleness to him on the Day of Resurrection."