Musnad Ahmad 25:565

Abu Sa’id narrated to us, Isma’il ibn Rabi’ah ibn Hisham ibn Ishaq ibn ‘Abdullah ibn Kinanah narrated to us, he said: I heard my grandfather, Hisham ibn Ishaq ibn ‘Abdullah, narrating from his father, he said: Al-Walid sent [someone] to ask Ibn ‘Abbas: "How did the Messenger of Allah, peace and blessings of Allah be upon him, perform the prayer for rain (Istisqa)?" He said: "The Messenger of Allah, peace and blessings of Allah be upon him, went out in a humble and lowly state. He came to the prayer ground and prayed two rak'ahs, just as he prays in the Fitr and Adha [prayers]."