Musnad Ahmad 329:10

Yazid narrated to us, he said: Ibn Abi Dhi'b informed us, from al-Zuhri, from 'Abbad ibn Tamim, from his uncle, who said: "I witnessed the Messenger of Allah, may Allah bless him and grant him peace, go out to pray for rain. He turned his back to the people and faced the Qibla, and he turned his cloak inside out, and he began to supplicate, and he prayed two rak'ahs, and he recited aloud."