It was narrated from Asma’ bint Yazid that :
the Messenger of Allah (ﷺ) said: “The Greatest Name of Allah is in these two Verses: (Al-Baqarah 163) And your Ilah (God) is One Ilah (God – Allah), La Ilaha Illa Huwa (none has the right to be worshipped but He), the Most Gracious, the Most Merciful.’ And at the opening of Surah Al ‘Imran (Ali ‘Imran 2).”
حَدَّثَنَا أَبُو بَكْرٍ، حَدَّثَنَا عِيسَى بْنُ يُونُسَ، عَنْ عُبَيْدِ اللَّهِ بْنِ أَبِي زِيَادٍ، عَنْ شَهْرِ بْنِ حَوْشَبٍ، عَنْ أَسْمَاءَ بِنْتِ يَزِيدَ، قَالَتْ قَالَ رَسُولُ اللَّهِ ـ صلى الله عليه وسلم ـ ” اسْمُ اللَّهِ الأَعْظَمُ فِي هَاتَيْنِ الآيَتَيْنِ {وَإِلَهُكُمْ إِلَهٌ وَاحِدٌ لاَ إِلَهَ إِلاَّ هُوَ الرَّحْمَنُ الرَّحِيمُ} وَفَاتِحَةِ سُورَةِ آلِ عِمْرَانَ ” .
Reference : Sunan Ibn Majah 3855
In-book reference : Book 34, Hadith 29
English translation : Vol. 5, Book 34, Hadith 3855