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
Ishaq bin Abdullah bin Kinanah narrates: Al-Walid bin Utbah sent me to Abdullah bin Abbas (may Allah be pleased with them both) and said: O my nephew, ask him how the Messenger of Allah (peace and blessings be upon him) performed the prayer for rain on the day he sought rain for the people. Ishaq said: I went to Ibn Abbas (may Allah be pleased with them both) and said: O Abu al-Abbas, how did the Messenger of Allah (peace and blessings be upon him) perform the prayer for rain on the day he sought rain for the people? He said: the Messenger of Allah (peace and blessings be upon him) went out humbly and in plain clothes, and he did in it just as he used to do on Eid al-Fitr and Eid al-Adha.
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10773