Sahih Muslim Book 4, Hadith Number 1959.

Chapter : Supplication in prayer for rain.

‘Ubaidullah b. Anas b. Malik heard (his father) Anas b. Malik as saying: A bedouin came to the Messenger of Allah (may peace be upon him) on Friday as he was (delivering the sermon on his) pulpit; and the rest of the Hadith is the same but with this addition: “I saw the cloud clearing just as a sheet is folded.”