Hadith on Allah: There is nothing greater than to testify that Allah is One

Abu Sa’id Al-Khudri reported: The Messenger of Allah, peace be upon him, said: Moses said, “My Lord, teach me something with which I can remember You and call upon You.” Allah said, “Say: there is no God but Allah.” Moses said, “My Lord, all Your servants say this.” Allah said, “O Moses, if the seven heavens along with their inhabitants besides Me and the seven earths were to be upon a scale, and the statement that there is no God but Allah, they would be outweighed by the saying: there is no God but Allah.”

[Sahih Ibn Hibban, Number 6218, Sahih]

عَنْ أَبِي سَعِيدٍ الْخُدْرِيِّ، عَنْ رَسُولِ اللهِ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ أَنَّهُ قَالَ‏ قَالَ مُوسَى‏ يَا رَبِّ عَلِّمْنِي شَيْئًا أَذْكُرُكَ بِهِ وَأَدْعُوكَ بِهِ‏ قَالَ‏ قُلْ يَا مُوسَى‏ لاَ إِلَهَ إِلاَّ اللَّهُ‏.‏ قَالَ‏ يَا رَبِّ كُلُّ عِبَادِكَ يَقُولُ هَذَا‏.‏ قَالَ‏:‏ قُلْ لاَ إِلَهَ إِلاَّ اللَّهُ‏ قَالَ‏ إِنَّمَا أُرِيدُ شَيْئًا تَخُصُّنِي بِهِ‏ قَالَ‏ يَا مُوسَى لَوْ أَنَّ أَهْلَ السَّمَاوَاتِ السَّبْعِ وَالأَرَضِينَ السَّبْعِ فِي كِفَّةٍ وَلاَ إِلَهَ إِلاَّ اللَّهُ فِي كِفَّةٍ مَالَتْ بِهِمْ لاَ إِلَهَ إِلاَّ اللَّهُ‏

ذِكْرُ سُؤَالِ كَلِيمِ اللَّهَ رَبَّهُ أَنْ يُعْلِمَهُ شَيْئًا يَذْكُرُهُ

صحيح ابن حبان