Allah Upanishad, or Allopanishad, is a book of dubious origin supposedly from Atharvaveda

Atharvaveda

The Atharvaveda is a sacred text of Hinduism and one of the four Vedas, often called the "fourth Veda"....

 and believed to be written during Mughal Emperor Akbar's reign.

Swami Dayananda Saraswati

Swami Dayananda Saraswati

Maharishi Dayanand Saraswati was an important Hindu religious scholar, reformer, and founder of the Arya Samaj, a Hindu reform movement. He was the first to give the call for Swarajya – "India for Indians" – in 1876, later taken up by Lokmanya Tilak...

's book Satyarth Prakash

Satyarth Prakash

The Satyarth Prakash is a 1875 book written by Swami Dayanand, the founder of Arya Samaj, and is considered its central text.-Overview:...

(The Light of Truth) argues that the Allopanishad is not part of the Upanishad

Upanishad

The Upanishads are philosophical texts considered to be an early source of Hindu religion. More than 200 are known, of which the first dozen or so, the oldest and most important, are variously referred to as the principal, main or old Upanishads...

canon. It does not even appear in the Atharvaveda. The book could have been written during the Mughal era (possibly during Akbar's reign).