The Prophet ﷺ said, "There are no days greater in the sight of Allah, nor any deeds more beloved to Allah, than those done during these days." He meant the first ten days of Dhul-Hijjah. "So increase in them your declarations of tahlil and tahmid."
عن النبي ﷺ قال: قال رسول الله ﷺ: مامن 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 told us, Abu Awana told us, from Musa bin Abu Ayesha, from Mujahid, from Ibn Abbas, and he mentioned it. Its isnad is sahih, and Musa bin Abu Ayesha is trustworthy and among the narrators found in all six major collections.