Musannaf Ibn Abi Shayba 21:243

Isa ibn Yunus narrated to us, from Abdullah ibn Abi Ziyad, from Shahr ibn Hawshab, from Asma bint Yazid, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The Greatest Name of Allah is in these two verses: 'And your God is one God; there is no deity except Him, the Entirely Merciful, the Especially Merciful' [Al-Baqarah: 163], And the opening of Surah Al-Imran: 'Alif-Lam-Mim. Allah - there is no deity except Him, the Ever-Living, the Sustainer of existence' [Al-Imran: 1-2]."