Aśvahṛdaya Akṣahṛdaya

1. Nala was able to drive the chariot so quickly because of his knowledge of the sacred chant Aśvahṛdaya 
2. Ṛtuparṇa was able to count the leaves and fruits because of his knowledge of the chant Akṣahṛdaya. 

They taught each other the sacred mantras.

http://paninformation.blogspot.com/search/label/NalopakhyanaParva