Hawzah News Agency – Imam Ali (peace be upon him) said, in his epistle to Malik al-Ashtar:

وإنْ عَقَدتَ بينَكَ وبينَ عدُوِّكَ عُقْدةً أو ألْبَسْتَهُ مِنكَ ذِمّةً فَحُطْ عهدَكَ بالوفاءِ وَارعَ ذمَّتَك بالأمانَةِ، واجعَلْ نفسَكَ جُنّةً دُونَ ما أعطَيتَ، فإنّهُ ليسَ مِن فرائضِ اللَّهِ شَي‏ءٌ النّاسُ أشَدُّ علَيهِ اجْتِماعاً مَع تَفرُّقِ أهوائِهم وتَشتُّتِ آرائهِم مِن تعظيمِ الوفاءِ بالعُهودِ.

'And if you make a covenant between yourself and your enemy or give him a surety [against life or property], then stand by your pact with loyalty and observe your liability with trustworthiness. Make yourself the shield for your word, for there is nothing from Allah's mandates that all people agree upon, despite their diverse opinions on everything else, as much as they do with regards to respecting the fulfilment of covenants.'

Source: Nahj al-Balagha, Letter 53