Abu Hurairah (May Allah be pleased with him) reported:The Messenger of Allah (ﷺ) 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. And he who utters: ‘Subhan-Allahi wa bihamdihi (Allah is free from imperfection and His is the praise)’ one hundred times a day, his sins will be obliterated even if they are equal to the extent of the foam of the ocean.”

[Al-Bukhari and Muslim].rophet Muhammad said: “Whoever says, `La ilaha illallah wahdahu la sharika lah, lahu’l-mulk wa lahu’l-hamd wa huwa `ala kulli shay`in qadir’ one hundred times in a day, will have (a reward) equivalent to freeing 10 slaves, 100 good deeds will be recorded for him and 100 bad deeds will be erased from him, and he will be protected from the devil for that day until evening comes. No one can do anything better than what he has done except one who says it more than that.”

On the authority of Abu Dharr (radiAllahu anhu), that the Prophet (sallAllahu alayhi wasallam) said –

“Whoever says ten times immediately after finishing the Fajr (Dawn) Payer, before moving from his place or talking to anyone,

“laa ilaaha illallaahu wahdahu laa shareeka lahu, lahul mulku wa lahul hamdu yuhyee wa yumeetu wa huwa alaa kulli shay’in qadeer – [ none has the right to be worshipped except Allaah alone having no partner, sovereignty is His and all praise is for Him and He brings life and He causes death and He has full power over everything,]”

ten good deeds will be recorded for him, ten sins will be eliminated from him, he will be upgraded ten degrees (in reward), he will be safeguarded from all kinds of harm all that day, he will be guarded against shaytan and no sin can overtake him on that day (and nullify his good deeds) except associating others in worship with Allaah. (At Tirmidhi no.3484 [5/515])

At-Tirmidhi commented on this hadeeth saying, “This is a hasan, saheeh hadeeth (i.e a good and authentic hadeeth)”.

Likewise it is an act of Sunnah to say dhikr ten times straight after Magrib salah too, according to a hadith related by Imaam Ahmad Hanbal (rahimahullah) on the authority of Umm Salamah (radiAllahu anha), hadith number 26430. Also stated in Sahih Ibn Hibban no. 2023.

Source : A Summary of Islamic Jurisprudence – Dr. Salih al-Fawzaan, Volume 1  , Chapter 10