Hisham bin Ishaq bin Abdullah bin Kinanah narrates from his father that al-Walid sent me to Abdullah bin Abbas (may Allah be pleased with them both) to ask him about the rain prayer of the Messenger of Allah (peace and blessings be upon him). So I came to him and said: We have been disputing in the mosque about the "salat al-istisqa'" (rain prayer) of the Messenger of Allah (peace and blessings be upon him). He said: No; rather, your brother's son al-Walid has sent you; he was the governor of Madinah at that time. Then Abdullah bin Abbas (may Allah be pleased with them both) said: The Messenger of Allah (peace and blessings be upon him) went out in plain clothes, humbly, with lowliness and earnest entreaty, and sat on the minbar. He did not deliver a sermon like this sermon of yours today; rather, he (peace and blessings be upon him) kept on supplicating, entreating and proclaiming the greatness of Allah. Then he (peace and blessings be upon him) led two rak'ahs, just as he used to lead in the two Eids.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6386