Musannaf Ibn Abi Shayba 27:364

Abu Mu'awiyah narrated to us, from al-A'mash, from Abu Salih, from Malik al-Dar, who said: He was the treasurer of 'Umar for food. He said: The people were struck by a drought during the time of 'Umar, so a man came to the grave of the Prophet (may Allah bless him and grant him peace) and said: "O Messenger of Allah, ask for rain for your nation, for they have perished." The man was visited in a dream and it was said to him: "Go to 'Umar, convey the greeting of peace to him, and inform him that you will be given rain. And say to him: 'Be prudent, be prudent.'" So he came to 'Umar and informed him, and 'Umar wept, then said: "O Lord, I do not spare any effort except for that which I am unable to do."