‘Umar did not seek the help of Allah by virtue of the status of al-‘Abbas (may Allah be pleased with him)
Question

Hadith: Anas ibn Malik (may Allah be pleased with him) narrated that when they suffered drought, ‘Umar (may Allah be pleased with him) would seek rain by virtue of al-‘Abbas ibn ‘Abd al-Muttalib, and he would say: “O Allah, we used to ask You for rain by virtue of our Prophet and You gave us rain. Now we ask You for rain by virtue of the paternal uncle of our Prophet, so give us rain” and they would be given rain. Is this saheeh? Is this evidence that it is permissible to seek the help of Allah by virtue of the status of the awliya’ (close friends of Allah)?.
Praise be to Allah.The hadith referred to by the questioner is a saheeh hadith which was narrated by al-Bukhari, but anyone who studies it will find that it is evidence that one should not seek help from Allah by virtue of the status of the Prophet (peace and blessings of Allah be upon him) or of anyone else, because tawassul (using a means to achieve a goal) and al-waseelah is the thing that helps you to achieve that goal. The waseelah referred to in this hadith (“we used to ask You for rain by virtue of our Prophet and You gave us rain. Now we ask You for rain by virtue of the paternal uncle of our Prophet, so give us rain” and they would be given rain) is seeking the help of Allah by virtue of the du`a of the Prophet (peace and blessings of Allah be upon him), as a man said: “O Messenger of Allah, our wealth has been destroyed and the roads are cut off, so pray to Allah to help us.” And because ‘Umar said to al-‘Abbas: “Get up, O ‘Abbas, and pray to Allah, so he prayed to Allah.” If this had come under the heading of seeking Allah’s help by virtue of a person’s status only, then ‘Umar (may Allah be pleased with him) would have sought the help of Allah by virtue of the status of the Prophet (peace and blessings of Allah be upon him) before doing so by virtue of the status of al-‘Abbas, because the status of the Prophet (peace and blessings of Allah be upon him) is greater before Allah than that of al-‘Abbas or anyone else. If this hadith came under the heading of seeking the help of Allah by virtue of status that it would have been more appropriate for the ameer al-mu’mineen ‘Umar (may Allah be pleased with him) to seek the help of Allah by virtue of the status of the Prophet (peace and blessings of Allah be upon him), not the status of al-‘Abbas ibn ‘Abd al-Muttalib. 
To sum up, there is nothing wrong with seeking the help of Allah by means of the du`a of a person who it is hoped will have his du`as answered because of his righteousness. The Sahaabah (may Allah be pleased with them) used to seek the help of Allah by means of the du`a of the Prophet (peace and blessings of Allah be upon him) for them. Similarly, ‘Umar sought the help of Allah by means of the du`a of al-‘Abbas ibn ‘Abd al-Muttalib (may Allah be pleased with him). So if you think a man is righteous and likely to have his du`as answered because his food, drink, clothing and housing are halaal, and because he is known to be a man of worship and piety, there is nothing wrong with asking him to pray to Allah for you and ask for what you like, on condition that this does not stir up self-admiration in this person whom you ask to make du`a for you. If it does stir up self-admiration, then it is not permissible for you to doom him by making this request of him, because that will harm him. 
I also say: This is permissible, but I do not recommend it. I think that each person should ask Allah by himself, without appointing any intermediary between him and Allah. That brings a greater hope and is nearer to fear of Allah. I also encourage anyone who asks his brother whose du`as he hopes will be answered to make du`a for him, to intend thereby to treat him kindly – i.e., the one who will make du`a – and not do it for the sake of meeting his own needs, because if he asks him for the sake of meeting his own needs, then it will become like asking him for money and the like, which is blameworthy. But if he intends thereby to benefit his brother who will make du`a by treating him kindly, then treating a Muslim kindly is something for which a person will be rewarded, as is well known – and that is better. And Allah is the source of strength. End quote. 
Majmoo’ Fatawa wa Rasaa’il Ibn ‘Uthaymeen (2/277). 
For more information on the hadith about ‘Umar seeking the help of Allah by virtue of al-‘Abbas (may Allah be pleased with them both) and the fact that this tawassul was by virtue of his du`a and not of his status, please see the book al-Tawassul by Shaykh al-Albani (may Allah have mercy on him), pp. 50-68.