The Prophet ﷺ said that the Greatest Name of Allah is contained in these two verses, "And your God is one God. There is no god but He, the Most Merciful, the Most Compassionate" [Baqarah 2:163], and the opening of Surah Al Imran, "Alif Lam Mim. Allah, there is no god but He, the Ever-Living, the Sustainer of all existence" [Al Imran 3:1–2].
حدثنا مسدد حدثنا عيسى بن يونس حدثنا عبيد الله بن أبي زياد عن شهر بن حوشب
أن النبي ﷺ قال اسم الله الأعظم في هاتين الآيتين {وإلهكم إله واحد لا إله إلا هو الرحمن الرحيم} وفاتحة سورة آل عمران {الم الله لا إله إلا هو الحي القيوم}
Its isnad is hasan. Tirmidhi reported it in the Book of Supplications, chapter "On the necessity of preceding the supplication with praise, glorification, and prayer upon the Prophet ﷺ before it...", hadith no. 3478, from the hadith of Isa bin Yunus, and he said, hasan sahih.