On 2 Kinds Of Knowledge.

[Abu Hafsat]

As-Salaam alaikum, 

Imam Darani (d. 830) was expelled from Syria for declaring he had a vision of the angels in which he was also spoken to by them. Abu Yazid al-Bistami (d. 874) was driven out of his hometown for claiming to have undergone a heavenly ascent (mi'eraj) similar in kind to that of the Prophet of Islam. 

As noted by Abu Hamid al-Ghazali, the Prophet, Sallallahu alaihi Wasallam, possessed 2 kinds of knowledge : one which he gave freely to the common lot of believers, and the other which he revealed only to select group of companions. 

The implication, of course, is that the latter was mystical or esoteric in nature. This was why Abu Huraira, may Allah be pleased with him, was quoted saying : "I have preserved from the Messenger of Allah, alaihi Salaat Wassalaam, 2 vessels (of knowledge), one from which I have freely given you, but were I to disseminate (what is in) the other, this throat would be cut off (laquti'a hazal-hulqum)".. 

And also Sayyadi Hudhayfa ibn al-Yamani was another companion to whom the Prophet, Sallallahu alaihi Wasallam, revealed not only the identities of the hypocrites, but more importantly, " the mysteries of knowledge (sara'iral ilm), subtleties of understanding (daqa'iq al-fah), and secrets of certainty (khafaya l-yaqin)". 

After Abu Hamid al-Ghazali, in his: 'Ihya Ulumiddin', to which you may refer for further reading.