Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) narrates that the Prophet (peace and blessings be upon him) said: "If you knew the measure of Allah Almighty's mercy (that is, its greatness and vastness), you would rely upon it (that is, upon the vastness of mercy) alone! (The narrator is in doubt about one word here.)"
Al-Bazzar recorded it, and its chain is hasan (good).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب البعث / 18560