Musannaf Ibn Abi Shayba 3:5963

Yazid ibn Harun narrated to us, from Ibn Abi Dhi'b, from al-Zuhri, from 'Abbad ibn Tamim, from 'Umar, who said: I witnessed the Prophet, may Allah bless him and grant him peace, go out to pray for rain. He faced the Qibla, turned his back [to the people], turned his cloak inside out, and prayed two rak'ahs, reciting aloud.