Hadith: Whoever says "there is no god but Allah, and Allah is Greatest," his Lord affirms it, saying: There is no god but Me, and I am Greatest

عن أبي سعيد الخدري وأبي هريرة رضي الله عنهما أَنهُما شَهِدَا عَلَى رسول اللَّه -صَلّى اللهُ عَلَيْهِ وسَلَّم- أَنه قالَ: «من قال: لا إله إلا الله والله أكبر، صَدَّقه ربه، فقال: لا إله إلا أنا وأنا أكبر. وإذا قال: لا إله إلا الله وحده لا شريك له، قال: يقول: لا إله إلا أنا وَحدِي لا شريك لي. وإذا قال: لا إله إلا الله له الملك وله الحمد، قال: لا إله إلا أنا لي الملك ولي الحمد. وإذا قال: لا إله إلا الله ولا حول ولا قوة إلا بالله، قال: لا إله إلا أنا ولا حول ولا قوة إلا بي» وكان يقول: «من قالها في مرضه ثم مات لم تَطْعَمْهُ النار».
[صحيح] - [رواه الترمذي وابن ماجه. ملحوظة: ذكر النووي الحديث بتغيير يسير في لفظه عما في كتب التخريج المسندة، كما أن للحديث عدة ألفاظ]

Abu Sa‘īd al-Khudri and Abu Hurayrah (may Allah be pleased with both of them) reported that they directly heard the Messenger of Allah (may Allah’s peace and blessings be upon him) saying: "Whoever says, 'there is no god but Allah, and Allah is Greatest,' his Lord affirms it, saying: 'There is no god but Me, and I am Greatest.' When one says, 'there is no god but Allah alone; He has no partner,' Allah says, 'there is no god but Me alone; I have no partner.' When one says: 'There is no god but Allah. To Him is the dominion and to Him is praise,' Allah says: 'There is no god but Me. To Me is the dominion, and to Me is praise.' When one says, 'there is no god but Allah, and there is no might nor power save with Allah,' Allah says: 'There is no god but Me, and there is no might nor power save with Me.' He (the Prophet) used to say: 'Anyone who says this in his illness then dies, the Fire will not consume him.'"
Sahih/Authentic. - [Ibn Maajah]

Explanation

Abu Hurayrah and Abu Sa‘īd al-Khudri (may Allah be pleased with both of them) reported that the Prophet (may Allah's peace and blessings be upon him) said that Allah affirms what His slave says. If one says, "there is no god but Allah, and Allah is Greatest," Allah says: "Indeed, there is no god but Me, and I am Greatest." When one says, "Allah is Greatest, and there is no might nor power save with Allah," Allah would affirm it. So, if a person said this Dhikr (There is no god but Allah, and there is no might nor power save with Allah) then died, the Fire would not consume him, i.e. that would be a reason not to be burnt in Hellfire. Therefore, every Muslim should memorize this Dhikr and say it when he is ill, so that Allah would grant him a good end for his life, Allah, the Almighty, Willing.