Musnad Ahmad 923:5

'Abdullah narrated to us, Muhammad ibn Ja'far al-Warkani narrated to us, Sharik informed us, from Abu Ishaq, from Sulayman, from Ubayy ibn Ka'b, who attributed it to the Prophet, may Allah bless him and grant him peace, and he said: "Two angels came to me. One of them said to the other: 'Recite it to him.' He said: 'Upon how many?' He said: 'A letter (harf).' He said: 'Increase him.' He said: 'Until it reached seven letters (ahruf).'"