A trustworthy narrator informed us that Mujahid (may Allah have mercy on him) used to say: Thunder (ra'd) is an angel, and lightning (barq) is the wings of the angel, which drive the clouds. Imam al-Shafi'i (may Allah have mercy on him) said: How closely what Mujahid (may Allah have mercy on him) said resembles the apparent meaning of the Qur'an!
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6475