It is narrated from Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) that Jibril (peace be upon him) came to the Prophet (peace and blessings be upon him) while he (peace and blessings be upon him) was at the pond (Adah) of Banu Ghifar, and said: "O Muhammad! Allah, the Mighty and Majestic, commands you to teach your Ummah to recite the Qur'an in one harf (mode of recitation)." Then he said: "In two harfs," until he reached seven harfs. "So whoever recites in any one harf of them, it is as he said."
Hadith Referenceالمعجم الكبير للطبراني / باب الألف / 532