Imaam Ahmad Ibn Hanbal (may Allah have mercy him) was asked:

Who can we ask after you?

He stated, “Ask ‘Abdul Wahhaab.” It was said, “Indeed he is not one who is vast in knowledge.” Imam Ahmad said, “Indeed he is a righteous man and the likes of him is given the success to be upon the truth.”

Source: Kitaab al-Wara’ of al-Marwadhee

Original Arabic

سئل الإمام أحمد بن حنبل رحمه الله : من نسأل بعدك ؟ فقال : سل عبدالوهاب ، فقيل له : إنه ليس له اتساع في العلم ! فقال الإمام أحمد : إنه رجل صالح ، مثله يوفق لإصابة الحق

[كتاب الورع للمروذي ص٧]

Translator’s note:

This is not to be used as a proof for some of the people of innovation who hold that as long as one follows the path of sufism, he doesn’t have to study because Allah will put knowledge into his heart. This is rejected from a number of angles:

1. The people of Sufism are not considered to be righteous; rather they are innovators.
2. ‘Abdul-Wahhab was a man from the people of the Sunnah. He is ‘Abdul-Wahhaab ibn ‘Abdil-Hakam al-Warraaq, one of the companions of Imaam Ahmad who took knowledge from him. He died in the year 250 or 251 Hijri.
3. It’s mentioned that he wasn’t a man who was vast in knowledge. This means he had knowledge but wasn’t on the level of Imaam Ahmad. However Imaam Ahmad directed the people to him due to ‘Abdul-Wahhaab being a man who feared Allah. And it is from the fear of Allah that when one does not know the answer to a question, he says, “I don’t know, Allah knows best.”
4. Imaam Ahmad mentioned “…the likes of him is given the success to be upon the truth.” This means that Allah will give him success to be in agreement with the Qur’an and the Sunnah. Those who say that Allah teaches their hearts (directly) and they don’t have to learn the religion, they’re in opposition to the truth. This is from the Shaytaan.
5. Allah said, “And fear Allah and Allah will teach you.” (Surah al-Baqarah:282) Also the Prophet mentioned: “Whoever traverses a path by which he seeks knowledge, Allah will make a path to paradise easy for him.” (Muslim)