It is narrated from Imam Thumamah bin Abdullah bin Anas (may Allah have mercy on him) that Sayyiduna Umar (may Allah be pleased with him) went out to pray for rain (istisqa') and took Sayyiduna al-Abbas (may Allah be pleased with him) out with him, and then, supplicating, he said: 'Allahumma inna kunna idha qahatna ala ahdi nabiyyina tawassalna ilayka bi-nabiyyika, Allahumma wa inna natawassalu ilayka bi-ammi nabiyyika' (O Allah, when we were struck by drought in the time of our Prophet (peace and blessings be upon him), we used to supplicate to You through Your Prophet (peace and blessings be upon him); O Allah, now we supplicate to You through the uncle of Your Prophet (peace and blessings be upon him)).
Hadith Referenceالمعجم الأوسط / باب الألف / 2437
Hadith Takhrij«أخرجه البخاري فى «صحيحه» برقم: 1010، 3710، وابن خزيمة فى «صحيحه» برقم: 1421، وابن حبان فى «صحيحه» برقم: 2861، والبيهقي فى «السنن الكبرى» برقم: 6520، والطبراني فى «الكبير» برقم: 84»