الكافي 2/2/30/7

7. A number of our people have narrated from Sahl ibn Ziyad and Ali ibn Ibrahim has narrated from his father all from ibn Mahbub from ibn Ri’ab from abu ‘Ubaydah from Thuwayr who has said the following: “I heard Ali ibn al-Husayn عليه السلام saying, ‘When the angels hear the believing person praying for his brother (in belief) in his absence or speak of him good, they say, “You are a good brother (in belief). You pray for him in his absence and speak good of him. Allah, the Most Majestic, the Most Holy, has granted you double of what you asked for him and He has praised you double of your praising him and you have excellence over him, but if they (angels) hear him speak evil of his brother (in belief), and pray against him, they will say, ‘You are a bad brother for him! Stop, O you whose sins and privacy are covered, reduce your burden. Praise and thank Allah, Who has covered you, and notice that Allah, the Most Majestic, the Most Holy, knows more about His servant than you do.’”