Musannaf Ibn Abi Shayba 22:129

Jarir narrated to us, from Mansur, from al-Hakam, who said: Mujahid, Abadah ibn Abi Lubabah, and some people used to review the copies of the Qur’an (Musahif). When the day arrived on which they intended to complete it, they sent for me and for Salamah ibn Kuhayl. They said: "We have been reviewing the copies of the Qur’an, and we intended to complete it today, so we wanted you to witness it with us." It used to be said: "When the Qur’an is completed, mercy descends at its completion," or "Mercy is present at its completion."