Al-Sha'bi narrates that Umar bin al-Khattab (may Allah be pleased with him) went out to ask for rain, and he did nothing but seek forgiveness, and then returned. It was said to him: We did not see you ask for rain. Umar (may Allah be pleased with him) said: I have asked for rain from the channels of the heaven from which rain comes down. Then he recited these verses: "Seek forgiveness of your Lord; indeed, He is ever Most Forgiving. He will send down the sky upon you in abundant rain." [Surah Nuh: 10, 11] and "O my people, seek forgiveness of your Lord, then repent to Him; He will send down the sky upon you in abundant rain." [Surah Hud: 52].
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6424