In Sahih Al-Bukhari it is recorded that the Messenger of Allah (Sallallaahu ‘Alayhi Wa Sallam) used to supplicate;
«اللَّهُمَّ لَكَ الْحَمْدُ غَيْرَ مَكْفِيَ وَلَا مُوَدَّعٍ وَلَا مُسْتَغْنًى عَنْهُ رَبَّنَا»
(O Allah ! All praise is due to You, without being able to sufficiently thank You, nor ever wish to be cutoff from You, nor ever feeling rich from relying on You; our Lord!)
It was reported that Prophet Dawud, peace be upon him, used to say in his supplication,”O Lord! How can I ever duly thank You, when my thanking You is also a favor from You to me” Allah the Exalted answered him, “Now, you have thanked Me sufficiently, O Dawud,” meaning, `when you admitted that you will never be able to duly thank Me.’
Source : Tafseer Ibn Katheer
Surah Ibrahim Ayah 33 : Describing Some of Allah’s Tremendous Favors
How our Prophet thanked Allaah, and how Dawud thanked Allaah
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