"The heart lied not in what he saw..." (Surah Najm 53:11)

مَا كَذَبَ الْفُؤَادُ مَا رَأَىٰ

“The (Prophet’s) heart lied not in what he saw.” (Surah Najm 53:11)

The fu’aad is the heart, and this means that the what the Prophet (sal Allahu alayhi wa sallam) saw with his eyes during the night journey and ascension, he also saw with certainty in his heart.

It could be that the eye sees something but the heart does not confirm it, and it could be the that heart sees something that the eye does not confirm. As for what the Prophet (sal Allahu alayhi wa sallam) saw during the night of Mi’raaj, it is the truth – in both his sight and in his insight, and that is why Allah ta’ala says: “The heart lied not in what he saw.” (Surah Najm 53:11)
 

http://muslimmatters.org/2010/03/31/tafseer-gems-from-shaykh-ibn-uthaymeen/