Allah (may He be Exalted) says: "And fulfil the Covenant of Allâh." [Surat Al An`am: 152]. (6) "This He has commanded you." (The Ten Commandments)
Keeping promises and fulfilling them are from the attributes of the believers, for Allah (Glorified and Exalted be He) marked them with these qualities and started with Himself. We have seen from previous Ayahs that Allah (Glorified and Exalted be He) does not break promises but fulfills them: "But those who fear their Lord (Allâh) and keep their duty to Him, for them are built lofty rooms; one above another under which rivers flow (i.e. Paradise). (This is) the Promise of Allâh: and Allâh does not fail in (His) Promise." [Surat Az-Zumar: 20].
In the story of Moses, Allah says: "So, did We restore him to his mother, that she might be delighted, and that she might not grieve, and that she might know that the Promise of Allah is true." [Surat Al Qasas: 13].
Accordingly, the believers, who believe in their Lord, keep to their promises and do not break them. As for hypocrites and disbelievers, their role model is Satan in breaking promises and appointments. Allah (may He be Exalted) says: "Then Mûsâ (Moses) returned to his people in a state of anger and sorrow. He said: "O my people! Did not your Lord promise you a fair promise? Did then the promise seem to you long in coming? Or did you desire that wrath should descend from your Lord on you, that you broke your promise to me (i.e. disbelieving in Allâh and worshipping the calf)?"" [Surat Taha: 86].
The previous Ayah was about the Jews, as for hypocrites, Allah (may He be Exalted) says: "And of them are some who made a covenant with Allah (saying): If He bestowed on us of His Bounty, we will verily, give Sadaqah (Zakat and voluntary charity in Allah's Cause) and will be certainly among those who are righteous. * Then when He gave them of His Bounty, they became niggardly [refused to pay the Sadaqah (Zakât or voluntary charity)], and turned away, averse. * So He punished them by putting hypocrisy into their hearts till the Day whereon they shall meet Him, because they broke that (covenant with Allah) which they had promised Him and because they used to tell lies." [Surat At-Tawbah: 75 - 77].
The Prophet (peace be upon him) said: "The signs of a hypocrite are three: when he talks he lies, Whenever he promises, he breaks his promise. Whenever he makes a covenant he proves treacherous." [Reported by Al Bukhari and Muslim]. The Five Compilers of Hadith (Imams Ahmad, Abu Dawud, Al-Tirmidhy, Al-Nasa'y and Ibn Majah) reported on the authority of `Abdullah ibn `Amr ibn Al `As who narrated that the Messenger of Allah (peace be upon him) said: "Whoever has the following four (characteristics) will be a pure hypocrite and whoever has one of the following four characteristics will have one characteristic of hypocrisy unless and until he gives it up. 1. Whenever he makes a promise, he breaks it. 2. Whenever he speaks, he tells a lie. 3. Whenever he makes a covenant, he proves treacherous. 4. Whenever he quarrels, he behaves in a very imprudent, evil and insulting manner." 
In another narration: "If you trust him, he proves to be dishonest" instead of: "When he makes a promise, he breaks it." Abu Dawud reported on the authority of `Abdullah ibn Abu Al Hamsa' who said: "I bought something from the Prophet (peace be upon him) before he received his Prophetic commission, and as there was something still due to him. I promised him that I would bring it to him at his place, but I forgot. When I remembered three days later, I went to that place and found him there. He said: You have vexed me, young man I have been here for three days waiting for you." This is the Prophet's commitment to promises, and who is keener than the Messenger of Allah (peace be upon him) with keeping to promises?
His Companions succeeded him in that quality, and Jabir ibn `Abdullah told that the Messenger of Allah (peace be upon him) said to him: "If the money of Bahrain comes, I will give you a certain amount of it. The Prophet (peace be upon him) had breathed his last before the money of Bahrain arrived. When the money of Bahrain reached, Abu Bakr announced: "Whoever was promised by the Prophet (peace be upon him) should come to us. I went to Abu Bakr and said: The Prophet (peace be upon him) promised me so and so. Abu Bakr gave me a handful of coins and when I counted them, they were five-hundred in number." [Reported by and Al Bukhari and Muslim].
So, Muslims have to keep to their promises and to adhere to the manners of Islam and what it called to. It was reported in the narration of Muslim: "The signs of the hypocrite are three, even if he fasts, prays and claims to be a Muslim." Then he mentioned: "Whenever he promises, he breaks his promise. Whenever he makes a covenant he proves treacherous. whenever he speaks, he tells a lie." How many people rebuke Muslims for not keeping their promises in this time; so you do not see a borrower fulfills his promise and renders the debt back in its due time. No craftsman fulfills his promises when he promises to finish work at a certain time. No one fulfills his promise by coming in a certain time but always comes late until this has become a dominant characteristic in most Muslims today, the enemies of Islam count it of Muslims' traits, and ignorant people count it as one of the qualities of Muslims although it is not. It is just one of the qualities of hypocrites, and a religious Muslim is innocent of breaking promises.
 Jami` Al Usul 9185.
 Ibid 9184.
 Ibid 9268.
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