Musannaf Ibn Abi Shayba 3:6071

Sahl ibn Yusuf narrated to us, from Humayd, from Anas, who said: It was asked, "Did the Messenger of Allah, may Allah bless him and grant him peace, raise his hands?" He said: "Yes. The people complained to him on a certain Friday and said, 'O Messenger of Allah, the rain has ceased, the land has become barren, and the livestock have perished.' He said: "So he raised his hands and supplicated until I saw the whiteness of his armpits."