Abd al-Malik bin Marwan said: Before I took charge of this affair (the responsibility of the caliphate), I used to sit with Sayyidah Barirah (may Allah be pleased with her) in Madinah, and Sayyidah Barirah (may Allah be pleased with her) used to say: O Abd al-Malik, I see in you qualities that make you fit to take charge of this Ummah. If you are given this responsibility, then beware of shedding blood, for I heard the Messenger of Allah (peace and blessings be upon him) say: "A man is kept back from the gate of Paradise merely because he shed unjustly a cupping-vessel's worth of a Muslim's blood, and then he cannot look at Paradise."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20896