Suuratul- Fajr Verse 2.
وَلَيَالٍ عَشْرٍ ﴿٢
2. By the ten nights (i.e. the first ten days of the month of Dhul-Hijjah)
Tafsir Ibn Kathir Volume 10, Page 468.
`The ten nights’ refers to the (first) ten days of Dhul-Hijjah. This was said by Ibn `Abbas, Ibn Zubayr, Mujahid and others among the Salaf and the latter generations. It has been confirmed in Sahih Al-Bukhari from Ibn `Abbas that the Prophet said,
«مَا مِنْ أَيَّامٍ الْعَمَلُ الصَّالِحُ أَحَبُّ إِلَى اللهِ فِيهِنَّ مِنْ هذِهِ الْأَيَّام»
(There are no days in which righteous deeds are more beloved to Allah than these days) meaning the ten days of Dhul-Hijjah. They said, “Not even fighting Jihad in the way of Allah” He replied,
«وَلَا الْجِهَادُ فِي سَبِيلِ اللهِ، إِلَّا رَجُلًا خَرَجَ بِنَفْسِهِ وَمَالِهِ ثُمَّ لَمْ يَرْجِعْ مِنْ ذلِكَ بِشَيْء»
(Not even Jihad in the way of Allah; except for a man who goes out (for Jihad) with his self and his wealth, and he does not return with any of that).
PS: I’m deviating from the monthly theme (O ye who believe) to celebrate the significance of the best 10 days of each year (similar to the best 10 nights in Ramadan) as they are mentioned in the Qur’an here in the last Juz.