Hadith: : .

عَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ أَنَّ رَسُولَ اللَّهِ صَلَّى اللهُ عَلَيْهِ وَسَلَّمَ، قَالَ:
«مَنْ قَالَ: لاَ إِلَهَ إِلَّا اللَّهُ، وَحْدَهُ لاَ شَرِيكَ لَهُ، لَهُ المُلْكُ وَلَهُ الحَمْدُ، وَهُوَ عَلَى كُلِّ شَيْءٍ قَدِيرٌ، فِي يَوْمٍ مِائَةَ مَرَّةٍ، كَانَتْ لَهُ عَدْلَ عَشْرِ رِقَابٍ، وَكُتِبَتْ لَهُ مِائَةُ حَسَنَةٍ، وَمُحِيَتْ عَنْهُ مِائَةُ سَيِّئَةٍ، وَكَانَتْ لَهُ حِرْزًا مِنَ الشَّيْطَانِ يَوْمَهُ ذَلِكَ حَتَّى يُمْسِيَ، وَلَمْ يَأْتِ أَحَدٌ بِأَفْضَلَ مِمَّا جَاءَ بِهِ إِلَّا أَحَدٌ عَمِلَ أَكْثَرَ مِنْ ذَلِكَ».
[صحيح] - [متفق عليه] - [صحيح البخاري: 3293]

Abu Hurayrah (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) said:
"Whoever says a hundred times in a day: "La ilāha illallāhu wahdahu la sharīka lah, lahu al-mulku wa lahu al-hamdu wa huwa ‘ala kulli shay’in qadīr" (There is no god but Allah. He is One, and He has no partner with Him; to Him belong the sovereignty and praise, and He is competent over all things); he will have a reward equivalent to that of emancipating ten slaves; a hundred good deeds will be added to his record; a hundred of his sins will be erased; and he will be shielded against the devil on that day till the evening; and no one will exceed him in doing good deeds except someone who says more than that."
[Authentic hadith] - [Narrated by Bukhari & Muslim] - [Sahih Bukhari - 3293]

Explanation

The Prophet (may Allah's peace and blessings be upon him) informed that whoever says: (There is no god) or true deity (but Allah. He is One, and He has no partner with Him) in His divinity, lordship, names, and attributes, (to Him belong sovereignty), control, and management, (and praise) for everything He creates and decrees, (and He is competent over all things) with no one to resist or oppose Him; and whatever He does not will, cannot be. Whoever recites this Dhikr (remembrance) in a day and repeats it one hundred times will receive a reward equivalent to that of freeing ten slaves; a hundred good deeds and ranks will be recorded for him in Paradise; a hundred sins will be erased from his record; it will serve as a shield, protection, and fortress from the devil and his misguidance for the entire day until evening (sunset); no one will come on the Day of Judgment with something better than what he brought, except someone who did more than this and exceeded it.

Benefits from the Hadith

1. The virtue of the word of Tawhid (the declaration of Allah's oneness) and the greatness of its reward.
2. The vast bounty of Allah Almighty upon His servants is shown in how He has legislated for them an easy Dhikr accessible to everyone, and has promised immense reward for it.
3. If a person says this declaration of Tawhid (there is no god but Allah) more than one hundred times in a day, he will receive the reward mentioned in the Hadīth for the first hundred, and he will also earn an additional reward for whatever he adds beyond that. This is not among the prescribed limits that are prohibited to exceed, nor is the extra repetition void of virtue or invalidated.
4. An-Nawawi said: The apparent wording of the Hadīth indicates that the reward mentioned in it is granted to whoever recites this Tahlīl (declaration of Tawhid) one hundred times during the day, whether the recitation is done consecutively or across different gatherings, or partly in the beginning of the day and partly at its end. However, it is better to recite it consecutively at the start of the day so that it serves as a protection for the person throughout the entire day.