Allahu Ta'ala Is Wajib-ul-wujud

Hadrat [1] Imam-i Rabbani says in the 17th letter of the third volume of his Maktubat:

Allahu ta'ala exists by His zat; His existence is of Himself. As He exists now, He has always been existing and will continue to exist. There cannot be nonexistence before or after His being. His existence is indispensable. That is, He is Wajib-ul-wujud (the Indispensable Being). There cannot be nonexistence at that rank. Allahu ta'ala is One. That is, He does not have a partner or a likeness.

He does not have a partner in being Wajib-ul-wujud, in being worshipped or in being worthy of worship. For having a partner, Allahu ta'ala has to be insufficient, dependent, which are defects and faults. There cannot be deficencies in wujub, and uluhiyyat. He is sufficient, independent. That is, He is by Himself. Then, there is no need for a partner or a likeness. And being unnecessary is a defect and is not compatible with wujub and uluhiyyat. As it can be seen, to think that He has a partner shows that each of the partners is insufficient. That is, to think that there is a partner exposes the fact that there cannot be a partner. That means to say that Allahu ta'ala does not have a partner. He is one.

[1] Hadrat: title of respect used before the names of great people like and Islamic scholars.