Musnad Ahmad 26:702

Abu Kamil narrated to us, Zuhayr narrated to us, Abu Hammam narrated to us, from Uthman ibn Hassan, from Falfalah al-Ju'fi, who said: I was alarmed among those who were alarmed regarding the copies of the Qur'an, so we entered upon Abdullah [ibn Mas'ud]. A man from the people said: "We did not come to you as visitors, but we came to you when this news startled us." He [Abdullah] said: "Indeed, the Qur'an was revealed to your Prophet, may Allah bless him and grant him peace, from seven gates, upon seven ahruf," or he said: "huruf (letters/modes). And indeed, the Book before it used to be revealed from one gate, upon one harf."