Amr bin Shu'ayb narrates from his father, and he from his grandfather (may Allah be pleased with him), that whenever the Prophet (peace and blessings be upon him) sought rain, he would say: 'Allahumma-sqi 'ibadaka wa baha'imaka, wanshur rahmataka, wa ahyi baladaka al-mayyit' (O Allah, give water to Your servants and Your livestock, spread Your mercy, and give life to Your dead land).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6441