Al-Agharr Abu Malik said: When Abu Bakr (may Allah be pleased with him) intended to appoint Umar (may Allah be pleased with him) as his successor, he sent for him, and when he came, Abu Bakr (may Allah be pleased with him) said: "I am calling you to a matter that is a burden for whoever takes charge of it. So, O Umar, fear Allah by obeying Him, and obey Him through fearing Him, for the God-fearing person is secure and protected. Moreover, this matter is something that none deserves except one who acts upon it. Whoever commands the truth yet acts upon falsehood, and enjoins what is right yet does what is wrong, is about to have his hope cut off and his deeds come to nothing. So if you are made the guardian of the people's affairs, then, if you are able, keep your hand dry of their blood, your stomach empty of their wealth, and your tongue clear of their honour; do so. And there is no power except with Allah."
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 37