Musnad Ahmad 25:858

Ya'qub narrated to us, Ibn Akhi Ibn Shihab narrated to us, from his uncle, who said: Ubayd Allah ibn Abd Allah ibn Utbah narrated to me, that Ibn Abbas narrated to him, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "Gabriel recited to me upon one letter, so I requested him to return [to it], and I did not cease asking him to increase it, and he would increase it for me, until it reached seven letters."