This ayah declares that it is most hateful in the sight of Allah that believers say what they do not do.
The commentary explains this as a strong rebuke of those who fail to fulfill their promises and words.
Scholars of the Salaf used this ayah to support the obligation of fulfilling one's promise, and it is connected to the Prophet's ﷺ warning that breaking a promise is a sign of hypocrisy.
رَبَّنَا آتِنَا فِي الدُّنْيَا حَسَنَةً وَفِي الْآخِرَةِ حَسَنَةً وَقِنَا عَذَابَ النَّارِ
Rabbana atina fid-dunya hasanah, wa fil-akhirati hasanah, wa qina 'adhaab an-nar
"Our Lord, give us good in this world and good in the Hereafter, and protect us from the punishment of the Fire."
— Hisnul Muslim — Quran 2:201 · Bukhari 6389 · Muslim 2690
وَقَالَ رَبُّكُمُ ٱدْعُونِىٓ أَسْتَجِبْ لَكُمْ ۚ إِنَّ ٱلَّذِينَ يَسْتَكْبِرُونَ عَنْ عِبَادَتِى سَيَدْخُلُونَ جَهَنَّمَ دَاخِرِينَ
"And your Lord says, "Call upon Me; I will respond to you." Indeed, those who disdain My worship will enter Hell [rendered] contemptible."
— Ghafir 40:60
"Whoever says SubhanAllah wa bihamdih one hundred times a day will have his sins forgiven even if they are as much as the foam of the sea."
— Sahih al-Bukhari 6405, Muslim 2691 · Sahih
حَسْبُنَا اللّٰهُ وَنِعْمَ الْوَكِيلُ
Hasbunallahu wa ni'mal-wakil
"Allah is sufficient for us and He is the best Disposer of affairs."
— Hisnul Muslim — Quran 3:173