Abbad bin Tamim al-Mazini (may Allah have mercy on him) narrates that he heard his uncle (may Allah be pleased with him), who was one of the Companions of the Messenger of Allah (peace and blessings be upon him), say: One day the Messenger of Allah (peace and blessings be upon him) went out to supplicate for rain. He (peace and blessings be upon him) turned his back to the people and, facing the qiblah, supplicated to Allah Almighty, and he also turned his cloak around; then he (peace and blessings be upon him) prayed two rak'ahs. Ibn Abi Dhi'b said in this hadith that he (peace and blessings be upon him) also recited in them. Ibn Wahb (may Allah have mercy on him) said: By this he means that he (peace and blessings be upon him) recited aloud.
Also, Imam Bukhari (may Allah have mercy on him) recorded in his Sahih, by this same route, the hadith he narrated from Adam bin Abi Iyas from Ibn Abi Dhi'b. In it he said: He (peace and blessings be upon him) led us in two rak'ahs and recited aloud in them. A similar report is narrated from Abu Nu'aym (may Allah have mercy on him), which he narrated from Ibn Abi Dhi'b, and Imam Muslim (may Allah have mercy on him) narrated this hadith through the chain of Abu al-Tahir. Ma'mar narrated it from al-Zuhri, first describing the manner of the prayer, then mentioning the turning of the cloak and the supplication.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6408