Sayyiduna Abd al-Rahman bin Awf (may Allah be pleased with him) said: The Prophet (peace and blessings be upon him) granted me and Sayyiduna Umar bin al-Khattab (may Allah be pleased with them both) such-and-such land. Sayyiduna al-Zubayr (may Allah be pleased with him) went to the family of Umar (may Allah be pleased with them) and bought their share from them. Then he came to Sayyiduna Uthman bin Affan (may Allah be pleased with him) and said: "Abd al-Rahman bin Awf claims that the Prophet (peace and blessings be upon him) granted him such-and-such land." He said: "His testimony is acceptable."
(b) Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) had said to Sayyiduna Abd al-Rahman bin Awf (may Allah be pleased with him): "You are, in our view, an upright man; what did you hear?"
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20394
Hasan (may Allah have mercy on him) said that the Messenger of Allah (peace and blessings be upon him) said: "When someone is asked about his brother, he has the choice: if he wishes he may remain silent, and if he wishes he may say something, and speak the truth."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب آداب القاضي / 20395