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