All those who are in positions of authority should fear Allah and treat all those under their supervision fairly and sincerely. Those in authority are not allowed to favor anyone at the expense of the other. The Prophet (peace and blessings be upon him) is reported to have said, “There is no man whom Allah causes to be appointed to a position of responsibility and he does not discharge his duties sincerely, but he will not even smell the fragrance of Paradise.” (Reported by Al-Bukhari)
Responding to the question, Dr. Salah Sultan, president of the American Center for Islamic Research, Columbus, Ohio, and member of the European Council for Fatwa and Research, states the following:
The Prophet (peace and blessings be upon him) stressed in many hadiths the importance of dealing justly with whomever the person is in charge of. In the famous authentic hadith it has been stated that the Prophet (peace and blessings be upon him) is reported to have said, “Each of you is a shepherd and each of you is responsible for his flock. The ruler is the shepherd of his people and is responsible for his flock. A man is the shepherd of his family and is responsible for his flock.” This means that every person who is in charge will be responsible for the people he has been entrusted with.
Another hadith states “Whoever is appointed a leader for ten people will come chained on the Day of Judgment, and only justice can help him get released.”
Another hadith in the Sunan of Abu Dawud warns: “Woe to the leaders from the Hellfire.” The author of `Awn al-Ma’bud fi Sharhi Abi Dawud comments, “Leadership is very important for people but this hadith is just a warning for leaders to deal fairly with people.”
Based on all of the above, it is mandatory on every manager or people in places of authority to deal fairly and justly with all his colleagues [and subordinates] and it is absolutely forbidden to favor anyone over another.