This is one of the main implications or principles of Wahdat Al-Wujud. Basically means every creature that the eye sees is Allah in essence. This concept is held by several so called soofee saints like Ibn Al-Faarid, Ibn ‘Arabi, Ibn ‘Sab’een, Al-‘Afeef At-Tilmisaani, Ibn Masheesh, An- Nabulusi and others. Many Soofis consider Ibn ‘Arabi as the symbol of the concept of Wahdat Al-Wujud.
Ibn 'Arabi considers the Soofee who knows of Allaah (‘Aarif billaah) as the one who “Sees Allaah in everything, rather he sees Him the essence of Everything," and that “He (Allaah) is the Essence of Existence” [Ibn ‘Arabi’s Fusoos al-Hikam [Beirut, Lebanon: Daar Al-Kitaab Al-‘Arabi, 1400 AH/1980 C.E.], p. 192.]
Historically, it started with Jahmiyyah and Mu'tazila who took their principles from Aristotle and Democritus.
Source: Sufism- Origin and Development