From Rigpa Wiki
When Shri Singha passed into parinirvana, Jnanasutra cried out to him and Shri Singha's last testament called the Seven Nails descended in Jnanasutra's hand. It is at that time that Shri Singha told him: "The texts of the innermost secret Nyingtik teachings are hidden inside a pillar of the Auspicious Ten-Thousand Gate Temple. Take them and practise them at the Bhasing Charnel Ground".
Vimalamitra later came to Bhasing to receive these instructions from Jnanasutra after he went to China and received the complete oral lineage from Shri Singha on the advice of Vajrasattva. Dakinis then told him to go to the Bhasing charnel ground to receive transmission from Jnanasutra. It is after this that he went to Tibet and introduced the Dzogchen teachings.