Musannaf Ibn Abi Shayba 35:380

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