Musannaf Ibn Abi Shayba 36:137

Muhammad ibn Abi ‘Adi narrated to us, from Dawud, from al-Sha’bi and Sa’id ibn al-Musayyib, regarding [the verse]: "And He sends down upon you from the sky, rain by which to purify you," they both said: "It was a light drizzle on the day of Badr."