My father told me that Walid bin Utba (Uthman said, Ibn Uqba, who was the governor of Madinah) sent him to Ibn Abbas to ask him about the prayer of the Messenger of Allah ﷺ for seeking rain.He said, "The Messenger of Allah ﷺ went out wearing simple clothes, humbly and beseechingly, until he reached the prayer ground." Uthman added, "Then he ascended the pulpit." Then both narrators agreed, "He did not deliver a sermon like these sermons of yours, but he continued in supplication, humble entreaty, and saying takbeer. Then he prayed two rakahs as he would pray on the Eid."
حدثنا النفيلي وعثمان بن أبي شيبة نحوه قالا حدثنا حاتم بن إسمعيل
أخبرني أبي قال أرسلني الوليد بن عتبة قال عثمان ابن عقبة وكان أمير المدينة إلى ابن عباس أسأله عن صلاة رسول الله ﷺ في الاستسقاء فقال خرج رسول الله ﷺ متبذلا متواضعا متضرعا حتى أتى المصلى زاد عثمان فرقى على المنبر ثم اتفقا ولم يخطب خطبكم هذه ولكن لم يزل في الدعاء والتضرع والتكبير ثم صلى ركعتين كما يصلي في العيد
Graded, its chain of transmission is hasan. Tirmidhi narrated it in the chapter on Prayer, in the section on what has been said regarding the rain prayer, hadith 558, from the hadith of Hatim bin Ismail with this chain, and he said, hasan sahih. Ibn Khuzayma graded it sahih, hadith 1405, as did Ibn Hibban, hadith 603.