1. No one denied at-tawassul before Ibn Taymiyyah
Imam as-Subki wrote in his book, Shifaa' is-saqaami fee ziyaarati khayr il-'anaam, what means "and no one from the salaf (Muslim from the 1st 3 hijri centuries) nor from the khalaf (those who came after the 1st 3 hijri centuries) preceded ibn Taymiyyah in denying at-tawassul; rather, he has made a statement that no knowledgable person has EVER said before him."
2. Ibn Taymiyyah was misguided
Ibn Hajar al-Haytami said in Hashiyat ubn Hajar 3ala sharh il-'eedaaH, what means, "and be not misguided by the denying that ibn Taymiyyah made regarding the sunnah of visiting Him (ie. the grave of the Prophet ), truly he was a slave (ibn Taymiyyah) whom Allah misguided, as al-3izz bin Jamaa3ah said, and as at-Taqi as-Subki mentioned at length in refuting him in an independent book (ie. a book whose sole purpose was to refute ibn Taymiyyah).
3. at-Tawassul by the Prophet is allowed during His life and after His death.
at-Tabarani mentions in al-Mu'jam al-Kabeer and also in al-Mu'jam as-Sagheer the Hadith of the blind man. In this Hadith Uthman bin Hunayf (Radiyallah anhu) mentions that a man had a need to see Uthman bin Affan (who was caliph at the time), but that Uthman would not look in his direction nor look into his affair. When this man met Uthman bin Hunayf, he explained the situation to him. So Uthman bin Hunayf told him,
"Go to the WuDu area, make wuDu, then pray 2 rak'ahs, then say 'O Allah, I ask you, and I turn to you by your Prophet Muhammad, the Prophet of Mercy, O Muhammad, I turn to my Lord through you regarding my affair, that it be decreed for me (ie. settled)' then go and I will go with you."So the man did what Uthman bin Hunayf said, then went to Uthman bin Affan's door, and the doorman took him by the hand and at sat him down with Uthman bin Affan (the caliph), who asked "what is your affair?" So the man mentioned his need, and Uthman ordered it handled then said, "I did not remember your affair until this very hour." Then the man left and said to Uthman bin Hunayf, "May Allah reward you with what's good. He didn't look into my affair nor turn in my direction until you spoke to him about me." Then Uthman bin Hunayf said,
And at-Tabarani said the Hadith is Saheeh. This Hadith has in it several proofs. First, that we may make at-tawassul during the Prophet's lifetime (the blind man). Second, that we may do so after His death (the man who needed to see Uthman the caliph). Third, that the Companions practiced at-tawassul. Fourth, even during the Prophet's lifetime , it was not a condition that the Prophet be present when you call to Him (the blind man was at the wudu area when he said "O Muhammad!"). As opposed to ibn Taymiyyah's opinion that at-tawassul was only permitted during His lifetime and while He was present.
"I swear by Allah, I did not speak to him, but I testify that a harmed man came to the Messenger of Allah
and complained of the hardship regarding his loss of sight. And [Muhammad
] said, 'If you wish, be patient, or if you wish I will make du'a for you.' The man said, 'O Messenger of Allah, my loss of sight is a hardship upon me since I have no one to lead me. So [Muhammad
] said to him, "Go to the WuDu area and make WuDu, and pray 2 rak'ahs then say, 'O Allah, I ask you...' (until the end of the du'a)
." And Uthman bin Hunayf said, "So the man did what He said, and I swear by Allah, we had not yet separated nor had we been sitting long when the man entered and he could see as if he had never been harmed."
4. at-Tawassul by pious Muslims is also permitted
Alaa' ad-deen al-Mardaawi, one of the most famous Hanbali scholars said in his book, al-Insaaf, what means, "and it is permissible to make at-tawassul by the and pious person according to the Saheeh position of the school. And it is said to be recommended (mustaHabb)."