In the name of Allah, the Beneficent, the Merciful
An-Nawawi recorded in his book Riyad us-Saliheen:
وَأَوْفُوا بِالْعَهْدِ إِنَّ الْعَهْدَ كَانَ مَسْئُولًا
Fulfill the covenant. Verily, the covenant will be questioned.
Surah Al-Isra 17:34
And Allah said:
وَأَوْفُوا بِعَهْدِ اللَّهِ إِذَا عَاهَدتُّمْ وَلَا تَنقُضُوا الْأَيْمَانَ بَعْدَ تَوْكِيدِهَا وَقَدْ جَعَلْتُمُ اللَّهَ عَلَيْكُمْ كَفِيلًا إِنَّ اللَّهَ يَعْلَمُ مَا تَفْعَلُونَ
Fulfill the covenant of Allah when you have taken it and do not break oaths after their confirmation while you have made Allah a witness over you. Verily, Allah knows what you do.
Surah An-Nahl 16:91
And Allah said:
يَا أَيُّهَا الَّذِينَ آمَنُوا أَوْفُوا بِالْعُقُودِ
O you who believe, fulfill the contracts.
Surah Al-Ma’idah 5:1
And Allah said:
يَا أَيُّهَا الَّذِينَ آمَنُوا لِمَ تَقُولُونَ مَا لَا تَفْعَلُونَ كَبُرَ مَقْتًا عِندَ اللَّهِ أَن تَقُولُوا مَا لَا تَفْعَلُونَ
O you who believe, why do you say what you do not do? Great it is in contempt to Allah that you say what you do not do.
Surah As-Saff 61:2-3
Abu Huraira reported: The Messenger of Allah, peace and blessings be upon him, said:
آيَةُ الْمُنَافِقِ ثَلَاثٌ إِذَا حَدَّثَ كَذَبَ وَإِذَا وَعَدَ أَخْلَفَ وَإِذَا اؤْتُمِنَ خَانَ
The signs of a hypocrite are three: whenever he speaks he lies, whenever he promises he breaks his promise, and whenever he is trusted he betrays his trust.
In another narration, the Prophet said:
وَإِنْ صَامَ وَصَلَّى وَزَعَمَ أَنَّهُ مُسْلِمٌ
Even if he fasts and prays and claims to be a Muslim.
Source: Sahih Bukhari 33, Grade: Muttafaqun Alayhi
Abdullah ibn Amr reported: The Messenger of Allah, peace and blessings be upon him, said:
أَرْبَعٌ مَنْ كُنَّ فِيهِ كَانَ مُنَافِقًا خَالِصًا وَمَنْ كَانَتْ فِيهِ خَلَّةٌ مِنْهُنَّ كَانَتْ فِيهِ خَلَّةٌ مِنْ نِفَاقٍ حَتَّى يَدَعَهَا إِذَا حَدَّثَ كَذَبَ وَإِذَا عَاهَدَ غَدَرَ وَإِذَا وَعَدَ أَخْلَفَ وَإِذَا خَاصَمَ فَجَرَ
There are four signs which make someone a pure hypocrite, and whoever has them has the characteristic of hypocrisy until he abandons it: when he speaks he lies, when he makes a covenant he proves treacherous, when he makes a promise he breaks it, and when he argues he proves wicked.
Source: Sahih Bukhari 34, Grade: Muttafaqun Alayhi
Jabir ibn Abdullah reported: The Messenger of Allah, peace and blessings be upon him, said to me:
لَوْ قَدْ جَاءَ مَالُ الْبَحْرَيْنِ قَدْ أَعْطَيْتُكَ هَكَذَا وَهَكَذَا وَهَكَذَا
When the revenues of Bahrain arrive, I will give you such-and-such.
But he passed away before the revenues were received. When they arrived during the time of Abu Bakr, he ordered an announcement, “Anyone whom the Messenger of Allah promised or owed something, he should come forward.” I went to him and said, “The Messenger of Allah promised me such-and-such.” He took a double handful from the money and gave it to me. I counted it and found that it was five hundred coins. Then Abu Bakr said to me:
Take twice as much as that.
Source: Sahih Bukhari 2174, Grade: Sahih
Success comes from Allah, and Allah knows best.