Hisham bin Ishaq bin Abdullah bin Kinanah narrated to us; he said: My father informed me, and I heard him narrating; he said: Al-Walid bin Uqbah (who was the governor of Madinah at that time) sent me to Sayyiduna Ibn Abbas (may Allah be pleased with them both) to ask him about the manner of the Messenger of Allah's (peace and blessings be upon him) rain prayer (salat al-istisqa'). So I came to him and said: We have disputed in the mosque about his (peace and blessings be upon him) rain prayer. He said: "No; rather, your nephew has sent you. If he has sent you, he has asked about something in which there is no harm." Then Sayyiduna Ibn Abbas (may Allah be pleased with them both) said: "He (peace and blessings be upon him) went out wearing plain clothes, humble and lowly, and imploring, until he (peace and blessings be upon him) sat on the pulpit. He did not deliver a sermon like this sermon of yours; rather, he (peace and blessings be upon him) kept supplicating, imploring and proclaiming the greatness of Allah Almighty, and he led two rak'ahs just as he used to lead them at Eid."
These are the words of the hadith of Ibrahim bin Musa, and the hadith of Yahya bin Yahya conveys the same meaning, except that he said: al-Walid bin Utbah. Abu Dawud al-Sijistani said: The correct (name) is Ibn Utbah.
The Shaykh (may Allah have mercy on him) said: This hadith gives the impression that his (peace and blessings be upon him) supplication was before the prayer, and Sufyan al-Thawri (may Allah have mercy on him) also narrated it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6402