It was narrated from Abu Hurayrah radiyAllaahu `anh that the Messenger of Allaah sallallaahu `alayhi wasallam said:
“Whoever says Laa ilaaha ill-Allaah wahdahu laa shareeka lah, lahu’l-mulk wa lahu’l-hamd wa huwa ‘ala kulli shay’in qadeer 100 times in a day, it will be as if he freed ten slaves, and one hundred hasanaat (good deeds) will be recorded for him, and one hundred sayi’aat (bad deeds) will be erased from him, and it will be a protection for him against the Shaytaan all that day until evening comes. No one can do anything better than that except one who does more.”
[Narrated by al-Bukhaari, 3119; Muslim, 2691]
“..as if he freed ten slaves”, and the reward for freeing a slave is described in this hadeeth:
“Whosoever frees a slave will get a reward to free one of his organs from the fire of hell for every organ of the slaves organs”
[Narrated in Saheeh Muslim]
Imagine that.. the Mercy of Allaah subhanahu wa ta`aala is so great! Look at the rewards He has put in a short statement, simply affirming His Oneness and glorifying Him. Just by reciting it 100 times you have added 100 good deeds (add that to the thousands you gained from reading soorah al-Kahf or any other soorah!), wiped out 100 bad deeds, guaranteed yourself protection from shaytaan for that entire day, and whats more, you have saved yourself from Hellfire. Allaahu Akbar!
I would like to add though, sometimes we recite while our hearts and minds are wandering elsewhere and so the words are uttered by our tongues but our hearts do not capture its true meaning. If you are going to recite this while typing away at a computer, or while sitting infront of a t.v. you are only cheating yourself. Say, when a husband praises his wife or expresses his love for her, he looks into her eyes, pays no attention to his surroundings and completely focuses on her. He makes her the centre of his attention for those few moments. Then how can we not give the same attention to the One Who created us, nurtured us, loved us and showered countless blessings upon us?
So when you are about to recite those words, set aside a few moments for Allaah and leave everything else. Remember, He isn’t the one who needs it. You do.