It is narrated from Talhah (may Allah have mercy on him): Marwan sent me to Ibn Abbas (may Allah be pleased with them both) to ask him about the manner of seeking rain. He said: The manner of the rain prayer is the same as that of the prayer of the two Eids, except that the Messenger of Allah (peace and blessings be upon him) turned his cloak around (reversed it), putting the right side onto the left and the left side onto the right, and prayed two rak'ahs. In the first rak'ah he (peace and blessings be upon him) said seven takbirs and recited the surah 'Glorify the name of your Lord, the Most High' [Surah al-A'la: 1], and in the second rak'ah he recited 'Has there come to you the report of the Overwhelming (Event)?' [Surah al-Ghashiyah: 1] and said five takbirs in it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6405