Majma al-Zawaid — Hadith 7128

Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) says: Sayyiduna Abbas (may Allah be pleased with him) said to me: O my son, the Commander of the Faithful calls for you, keeps you close, and consults you along with the Companions of the Prophet (peace and blessings be upon him). So remember three things from me: beware lest he ever encounter a lie from you, do not disclose any secret before him, and do not backbite anyone in his presence. The narrator Amir says: I said to Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both): Each one of these is better than a thousand. He said: Each one of these is better than ten thousand.
Al-Tabarani recorded it; its chain contains a narrator, Mujalid bin Sa'id, whom al-Nasa'i (may Allah have mercy on him) and others declared thiqah (trustworthy), while a group declared him da'if (weak).