Hadith Qudsi 52
52) (One day) Abu Huraira and Abu Sa'id were sitting with Allah's Messenger who said: When a slave says: There is no God but Allah and Allah is the Most-Great. Allah remarks: My slave has spoken the truth. Indeed, there is no god except Me and I am All-Great. When he says:There is no God .but Allah and He is the only One. Allah remarks: My slave has spoken the truth. Indeed, there is no God except Me and I am the One. When he says: There is no God but Allah and there is no associate with Him. Allah remarks: My slave has spoken the truth. There is no God except Me and I have no partner. When he says: There is no God but Allah. To Him belongs the sovereignty and to Him is due all praise. Allah remarks: My slave has spoken the truth. There is no God except Me. To Me belongs the kingdom and to Me is due all praise. When he says: There is no God but Allah and there is no power or might except with Him. Allah remarks: My slave has spoken the truth. There is no God except Me and the power and might belong to Me.
(Its chain of narratirs is sound and reported by Ibn Majah, Tirmidhi and Ibn Hibban).
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In Islam, remembrance of Allah HJ}-S is very much recommended, anywhere,any time, and in any condition-Such words.em are very easy to pronounce but very valuable in reward.