It is narrated from Talhah bin Abdullah bin Awf (may Allah have mercy on him): I asked Sayyiduna Ibn Abbas (may Allah be pleased with them both) about the manner of the rain prayer, and he said: "It is like the manner of Eid. The Messenger of Allah (peace and blessings be upon him) went out seeking rain. He (peace and blessings be upon him) led two rak'ahs without adhan or iqamah and said twelve (12) takbirs in them, seven in the first and five in the second, and recited aloud. Then he (peace and blessings be upon him) turned away and delivered a sermon. Then he faced the qiblah, turned his cloak around, and supplicated for rain."
This Muhammad bin Abd al-Aziz is not strong, but he is strengthened by the supporting reports before it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6406