In the Jewel Ornament of Liberation (chapter on prajnaparamita, p. 251-255) Gampopa writes,
If the meaning of emptiness does not dwell within the mind, we cannot attain liberation by means of the other virtues.
When one is endowed with the meaning of emptiness, there is not a single thing which in not included in this path.
Cultivation of bodhicitta is also included. Visualization of the deity and the recitation of mantras are also said
to be complete when this [the realization of emptiness] exists. Performing fire-offering rituals is also included in this. Even the path—the six perfections—is complete in this. Doing prostrations is also included in this. It is also offering if one has this. Again, if one is endowed with this, it is also the very purification of evil deeds. The guarding of moral ethics and samaya is also included in this meaning. Listening, reflecting, and meditating are also encompassed in this meaning. Torma cakes and daily Dharma rituals are also included in this meaning.
In that case, if all these are included in meditating on only the essence or the mind-as-such, why do there appear teachings on so many graduated methods? It is for the purpose of leading all those sentient beings of little fortune, who are ignorant in the ultimate nature.
"While teachers of the middle way, mind only, transcendent wisdom, mantra, and other schools may have their own assertions, the fulfillment of those intentions is the same. There is not a single thing that is not contained within mind."
(Gampopa to Düsum Khyenpa, in "The First Karmapa", KTD Pub, p254)
“If you recognize the world of appearance and existence as the mind, realize the mind itself as empty, and have no grasping at the superiority of your realizations — this is the ultimate view."
(Chegom Dzongpa, in "The Book of Kadam", Wisdom Pub, p609)