Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrates that the Messenger of Allah (peace and blessings be upon him) went out, when he sought rain, in humility, submissiveness and lowliness, just as he (peace and blessings be upon him) used to do at the two Eids. Abdullah bin Yusuf al-Tinnisi (may Allah have mercy on him) narrated it from Isma'il bin Rabi'ah (may Allah have mercy on him) with the same meaning.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6404