Sayyiduna Zayd bin Thabit (may Allah be pleased with him) narrates: I used to write down the revelation in the presence of the Messenger of Allah (peace and blessings be upon him). His (peace and blessings be upon him) breathing would become heavy and he would sweat profusely, with drops like pearls; then this state would lift from him. I would write while he (peace and blessings be upon him) dictated to me, and I would not yet have finished when the heaviness would come over him (peace and blessings be upon him) again. When I finished, he (peace and blessings be upon him) would say: "Read." So I would read it back to him (peace and blessings be upon him), and if anything had been left out of it, he (peace and blessings be upon him) would have it corrected.
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4869