It is narrated from Sayyiduna Anas (may Allah be pleased with him) that Umar (may Allah be pleased with him) went out to pray for rain, and he took al-Abbas (may Allah be pleased with him) out with him to supplicate, and he would say: "O Allah, whenever we were struck by drought in the time of our Prophet (peace and blessings be upon him), we would supplicate to You through the means (wasilah) of our Prophet (peace and blessings be upon him), and now we supplicate to You through the means of the uncle of our Prophet (peace and blessings be upon him) (al-Abbas, may Allah be pleased with him)."
Hadith Referenceالمعجم الكبير للطبراني / العشرة / 83