It is narrated from Sayyiduna Anas (may Allah be pleased with him) that whenever they were struck by drought, Sayyiduna Umar bin al-Khattab (may Allah be pleased with him) would supplicate with Sayyiduna al-Abbas bin Abd al-Muttalib (may Allah be pleased with him) and say: 'Allahumma inna kunna natawassalu ilayka bi-nabiyyina fa-tasqina, wa inna natawassalu ilayka bi-'ammi nabiyyina fasqina' (O Allah, we used to come to You through our Prophet (peace and blessings be upon him) as a means, and You would give us rain; today we come to You through the uncle of our Prophet (peace and blessings be upon him) as a means, so give us rain). And they would be given rain. Imam Bukhari (may Allah have mercy on him) narrated it in his Sahih from al-Hasan bin Muhammad al-Za'farani, and he said: This report is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) without any doubt. He says that his name dropped out of the book of our (Shaykh) Abu Muhammad.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب صلاة الاستسقاء / 6427