Sunnah.com reference : Book 2, Hadith 449English translation : Book 2, Hadith 516Arabic reference : Book 2, Hadith 518
Narrated [Anas (RA)]:
When they experienced drought 'Umar bin al-Khattab (RA) used to seek rain by asking al-'Abbas bin 'Abdul Muttalib (RA) to supplicate Allah for rain. He ('Umar) would say: 'O Allah, we used to ask our Prophet (RA) to supplicate to You for rain, and You would give us rain. We are now asking our Prophet's uncle to supplicate to You for rain, so give us rain." They would then be given rain.

[Reported by al-Bukhari].
وَعَنْ أَنَسٍ; { أَنَّ عُمَرَ ‏- رضى الله عنه ‏- كَانَ إِذَا قَحِطُوا يَسْتَسْقِي بِالْعَبَّاسِ بْنِ عَبْدِ اَلْمُطَّلِبِ.‏ وَقَالَ: اَللَّهُمَّ إِنَّا كُنَّا نَسْتَسْقِي إِلَيْكَ بِنَبِيِّنَا فَتَسْقِينَا, وَإِنَّا نَتَوَسَّلُ إِلَيْكَ بِعَمِّ نَبِيِّنَا فَاسْقِنَا، فَيُسْقَوْنَ }  رَوَاهُ اَلْبُخَارِيُّ  1‏ .‏‏1 ‏- صحيح.‏ رواه البخاري (1010)‏.‏