Musannaf Ibn Abi Shayba 21:557

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