Sahl bin Sa'd (may Allah be pleased with him) said: I heard Sayyiduna Zayd bin Thabit (may Allah be pleased with him) saying: "When revelation came down upon the Messenger of Allah (peace and blessings be upon him), a heaviness would come over him (peace and blessings be upon him), and sweat would run from his (peace and blessings be upon him) forehead like pearls, even if it was in the cold season." (These are the words of Uqbah bin Mukram's narration.)
Hadith Referenceالمعجم الكبير للطبراني / باب الزاي / 4770