Ishaq bin Abdullah bin Kinanah reported that his father sent him to Ibn Abbas to ask him about the prayer of the Messenger of Allah ﷺ for rain. Ibn Abbas said, "The Messenger of Allah ﷺ went out humbly, modestly, and in plain garments. He did not deliver a sermon like this sermon of yours, but he prayed two rakahs."
أخبرنا إسحق بن منصور ومحمد بن المثنى عن عبد الرحمن عن سفيان عن هشام بن إسحق بن عبد الله بن كنانة عن أبيه قال أرسلني فلان إلى
عن صلاة رسول الله ﷺ في الاستسقاء فقال خرج رسول الله ﷺ متضرعا متواضعا متبذلا فلم يخطب نحو خطبتكم هذه فصلى ركعتين
[Its chain of transmission is hasan] Ibn Majah narrated it in the Book of Establishing the Prayers, chapter on what has been said regarding the prayer for rain, hadith no: 1266, from the hadith of Sufyan Thawri. Abu Dawud narrated it, hadith no: 1165, and Tirmidhi, hadith no: 558, 559, from the hadith of Hisham bin Ishaq, through him, and he is hasan al-hadith. Tirmidhi said, "Hasan sahih." It is also found in Al-Kubra, hadith no: 1808, and Ibn Khuzayma graded it sahih, hadith no: 1405, as did Ibn Hibban, hadith no: 603, and others besides them.