The Prophet ﷺ said, "There are no days greater in the sight of Allah, nor any in which deeds are more beloved to Him, than these days," meaning the ten days of Dhul-Hijjah, "so increase in them the saying of La ilaha illa Allah and al-Humayd lillah."
عن النبي ﷺ قال: قال رسول الله ﷺ: ما من أيام أعظم عند الله، ولا العمل فيهن أحب إلى الله من هذه الأيام، فأكثروا فيها التهليل والتحميد يعني أيام العشر.
Abu Awana narrated it in his Musnad (3024) from Abu Yahya Abdullah bin Ahmad bin Abu Maysara, who said, Abdul-Hameed bin Ghazwan Basri narrated to us, Abu Awana narrated to us, from Musa bin Abu Ayesha, from Mujahid, from Ibn Abbas, and he mentioned it. Its chain of transmission is sahih, and Musa bin Abu Ayesha is trustworthy and one of the narrators found in the collections of all the major hadith compilers.Ahmad (5446) and Abd bin Humayd (807) both narrated it as well, from the hadith of Abu Awana, who said, Yazid bin Abu Ziyad narrated to us, from Mujahid, with the same chain and the same wording.Yazid bin Abu Ziyad is the Hashimi, their freed client, and he is daif, but he has been corroborated in the first chain of transmission.