It is narrated from Ubadah bin Samit (may Allah be pleased with him) on the authority of Ubayy bin Ka'b (may Allah be pleased with him) that the Noble Prophet (peace and blessings be upon him) said: The Noble Qur'an was revealed upon seven letters.
Hadith Referenceمسند احمد / مسند الأنصار رضي الله عنهم / 21091