Reference : Hisn al-Muslim 260

وَقَالَ صلى الله عليه وسلم: يَا عَبْدَ اللَّهِ بْنَ قَيْسٍ أَلاَ أَدُلُّكَ عَلَى كَنْزٍ مِنْ كُنُوزِ الْجَنَّةِ؟ فَقُلْتُ: بَلَى يَا رَسُولَ اللَّهِ، قَالَ: قُلْ (لاَ حَوْلَ وَلاَ قُوَّةَ إِلاَّ بِاللَّهِ).
Allah's Messenger (ﷺ) said: "O Abdullah bin Qais, should I not point you to one of the treasures of Paradise?" 
I said, "Yes, O Messenger of Allah." 
So he told me to say:
Lā ḥawla wa lā quwwata 'illā billāh.

Allah's Messenger (ﷺ) said: "O Abdullah bin Qais, should I not point you to one of the treasures of Paradise?" 
I said, "Yes, O Messenger of Allah." 
So he told me to say:
There is no power and no might except by Allah.

Reference:
Al-Bukhari, cf. Al-Asqalani, Fathul-Bari 11/213, Muslim 4/2076.

Chapter: Excellence of Tasbih, Tahmid, Tahlil, and Takbir