I basically wanted the proof of all is ONE and Prakrithi and Purusha are the same.
I am giving the proof from Sastra and Aaptavakya or Mahajanavakya as these two are accepted as Pramana or proof in our tradition.
Purusha is known as Shiva(or Vishnu) and Prakriti as Shakti (or Devi). These two are the same:
(a) Shakti-Shaktimatorbhedah Shaive jatu na varnyate/na shivah shakti-rahito na shaktir vyatirekini (Shiva-Drishti.,3/2 and 3/3)
(b) sridevi prakriti proktaa keshavah purushah smritah/na vishnunaa vinaa devi na harih padmajaam vina (hayasirsha-pancharaatra-vachana)
Meaning : (a)According to the of Shaiva Philosophy, there is no difference between Shakti and Shaktimaan. Shiva is never devoid of Shakti and Shakti is never devoid of Shiva. (Reference: Sastramulak Bharatiya Shaktisadhana, Upendrakumar Das, RMIC, vol 1, page 318)
(b) Prakriti is Sri Devi and Purusha is Keshava. Vishnu is never without Devi and Devi Padmajaa is never without Vishnu.
This is also referred to as Shakti-Shaktimator abhedah meaning there is no difference between Shakti or Prakriti and Shaktimaan or Purusha.
This same concept is also there in Devi-Bhagavatam 1/8/34, 40-41 (quoted in the commentary of Tripuramahopabishad by Bhaskara Raya
Evam Sarvagataa shaktih saa brahmeti vivichyate
Meaning The All-pervading Shakti is One with Brahman
Sri Ramakrishna explains as
Brahman is one with Shakti. Like Milk is one with its whiteness.Still water and wavy water are the same. A snake can be still (Brahman) and moving (Shakti).Agni or Fire is one with its dahika-Shakti or burning power.
As Shiva-Purana says (5/9-12):
yathaa Shivas tathaa Devi yathaa Devi tathaa shivah/nanayor anratam vidyat chandra-chandrikayoriva//
meaningn: Shiva is Shakti and Shakti is Shiva. They are one as the moon and its light.
(Reference: Sri Sri Ramakrishna-Kathamrita, Sri M (compiled) Udbodhan