The Prophet ﷺ said that the greatest name of Allah is found in these two verses, "And your God is one God. There is no deity except Him, the Most Merciful, the Especially Merciful" [Baqarah 2:163], and the opening of Al Imran, "Alif, Lam, Meem. Allah, there is no deity except Him, the Ever-Living, the Sustainer of existence" [Al Imran 3:1–2].
حدثنا علي بن خشرم حدثنا عيسى بن يونس عن عبيد الله بن أبي زياد القداح عن شهر بن حوشب
أن النبي ﷺ قال اسم الله الأعظم في هاتين الآيتين {وإلهكم إله واحد لا إله إلا هو الرحمن الرحيم} وفاتحة آل عمران {الم الله لا إله إلا هو الحي القيوم}
[Its isnad is hasan] Abu Dawud also narrated it, no. 1496, and Ibn Majah, no. 3855, from the hadith of Isa bin Yunus, through this same chain. Shahr bin Hawshab is a narrator whose hadith is hasan, see Nail al-Maqsud, no. 134, and others.