Abu Hurairah (May Allah be pleased with him)reported: The Messenger of Allah (PBUH) said, "He who utters a hundred times in a day these words: `La ilaha illallahu, wahdahu la sharika lahu, lahul-mulku wa lahul-hamdu, wa Huwa `ala kulli sha'in Qadir (there is no true god except Allah. He is One and He has no partner with Him; His is the sovereignty and His is the praise, and He is Omnipotent),' he will have a reward equivalent to that for emancipating ten slaves, a hundred good deeds will be recorded to his credit, hundred of his sins will be blotted out from his scroll, and he will be safeguarded against the devil on that day till the evening; and no one will exceed him in doing more excellent good deeds except someone who has recited these words more often than him.)
[Al-Bukhari and Muslim].

This sunnah is great, and its reward is unimaginable; and the Messenger of Allah declared that Tahlil is the best ever in his words and the words of his fellow prophets.

Al-Tirmidhi related on the authority of `Abdullah ibn Amr ibn al-Aas that the Messenger of Allah said: (The best thing I and the prophets before me have said: (there is no true god except Allah. He is One and He has no partner with Him; His is the sovereignty and His is the praise, and He is Omnipotent)."

https://islamstory.com/en/artical/3407664/sunnah-tahlil