Ishaq bin Abdullah bin Kinanah narrates: One of the governors sent me to Ibn Abbas (may Allah be pleased with them both) to ask him about the prayer for rain (istisqa'). He said: Who sent you? I said: So-and-so. He said: What prevented him from coming to me himself and asking? the Messenger of Allah (peace and blessings be upon him) went out humbly, imploring, and in plain clothes; he (peace and blessings be upon him) did not deliver this sermon of yours, and he prayed two rak'ahs just as the prayer is offered on Eid.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10772