what is Brotherhood ?

“Once, a man from Basra got up and said to Ameerul Mo’mineen, ‘Would you tell us about brothers?’
Imam Ali said, ‘Brothers are two kinds; brothers of trust and brothers of smile. As for the brothers of trust, they are like the hand, the wing, the kin and the money to you. If there is trust between you and your brother, you give him from your money, help him with your hand, make friends with his friends, be enemy to his enemy, keep his secrets, support him and show his good qualities. O asking man, know that brothers of trust are rarer than the red sulfur. And as for the brothers of smile, you get your pleasure from them. Do not cut your relation with them and do not ask them for more from their consciences. Smile at them and be courteous to them as long as they do that to you.’ ( Wassa'il ash-Shia, vol. 8 p.58. )

Reference : The Life of Imam Muhammad Al-Jawad