Musnad Ahmad 25:1472

Waki’ narrated to us, Sufyan narrated to us, from Hisham ibn Ishaq ibn ‘Abdullah ibn Kinanah, from his father, who said: A commander from among the commanders sent me to Ibn ‘Abbas to ask him about the prayer during the request for rain (Istisqa’). Ibn ‘Abbas said: "What prevented him from asking me? The Messenger of Allah (peace and blessings of Allah be upon him) went out humble, dressed in simple clothes, showing submissiveness, walking with ease, and supplicating. He prayed two rak'ahs just as he prays on the day of the Eid, and he did not deliver a sermon like this sermon of yours."