Imam Malik {May Allah have mercy upon him} said:
I know many Hadith and had a great deal of knowledge that I never spoke about nor did I mention it. And that was al ‘Ilm ul~Baatin, = the knowledge of the heart.
{Kitab al-Muwafaqat fi Usul as-Shariah ~ The Reconciliation of the Fundamentals of Islamic Law, Volume 4, pg. 36}