Musnad Ahmad 926:5

Abdullah narrated to us, Muhammad ibn Sulayman al-Asadi Luwayn narrated to us, al-Hasan ibn Muhammad ibn A'yan narrated to us, Umar ibn Salim al-Aftas narrated to us, from his father, from Zubayd, from Abd al-Rahman ibn Abi Layla, from Ubayy ibn Ka'b: "Gabriel came to the Prophet, may Allah bless him and grant him peace, while he was at the watering place of Banu Ghifar. He said: 'O Muhammad, indeed Allah commands you to recite the Quran in one mode.' He continued to increase it for him until it reached seven modes."