Hawzah News Agency – Imam Sadiq peace be upon him says:

المُؤمنُ أخو المؤمنِ، عَينُهُ ودَليلُهُ، لا يَخونُهُ، ولا يَظْلِمُهُ، ولا يَغُشُّهُ، ولا يَعِدُهُ عِدَةً فيَخْلِفَهُ.

'A believer is the brother of another believer, his eye and his guide; he does not betray him, nor wrong him, nor deceive him, and nor does he rescind a promise he made to him.'

Source: al-Kafi, v. 2, p. 166, no. 3