Musnad Ahmad 329:22

Sufyan narrated to us, from Abdullah ibn Abi Bakr ibn Muhammad ibn Amr ibn Hazm, who heard Abbad ibn Tamim, from his uncle: "That the Messenger of Allah (peace and blessings of Allah be upon him) went out to the prayer ground (musalla) to seek rain. He faced the Qibla, turned his cloak inside out, and prayed two rak'ahs." Sufyan said: Turning the cloak means making the right side the left side, and the left side the right side.