The following text is extracted from Shaykh Ibn Ata Allah al-Iskandari’s (r.a) The Key to Salvation and the Lamp of Souls.
The fourth invocation is Allah. It is called the single invocation, because the invoker contemplates the Majesty and Sublimity of God, while being extinguished from himself. God Most High has said,
It is related that al-Shibli(r.a) was asked by a man, ‘Why do you say Allah and not La ilaha illa Allah (there is no divinity but God)?’ So al-Shibli(r.a) answered; ‘Because Abu Bakr(r.a) gave all his wealth to the point where not a thing remained with him. Then he took off a garment in front of the Prophet (s.a.w). So the Messenger of God (s.a.w) said, ‘What did you leave for your family?’ He answered, ‘Allah‘. Likewise I say Allah.’
Then the questioner said, ‘I want a higher explanation than this’. So al-Shibli(r.a) said, ‘I am embarrassed to mention an expression of negation in His presence, while everything is His light.’ Then the man said, ‘I want a higher explanation than this’. Al-Shibli (r.a) answered, ‘I am afraid that I will die during the negation of the phrase before reaching the affirmation.’ The questioner then again said, ‘ I want a higher explanation than this’. So al-Shibli (r.a) said, ‘God Most High said to His Prophet (s.a.w):
“…………….Say Allah! Then leave them to their vain talk”- Quran(6:91)
Then the young man got up and let out a shriek. Al-Shibli (r.a) said ‘Allah’. He screamed again; and al-Shibli (r.a) said ‘Allah’, then he screamed a third time and died, may God Most High have mercy upon him! The relatives of the young man gathered together and grabbed al-Shibli (r.a), charging him with murder. They took him to the Caliph and were given permission to enter, and they accused him of murder. The Caliph said to al-Shibli (r.a), ‘What is your response?’ He answered, ‘A young soul yearned, then wailed and aspired, then screamed, then was summoned, then heard, then learned, then answered. So what is my crime?’ The Caliph shouted, ‘Let him go!’
The reason for this teaching on the simple invocation is because God is the goal and the most worthy of being invoked; because the invoker of ‘There is no divinity but God (La ilaha illa Allah) might die between the negation and the affirmation; because saying Allah only is easier on the tongue and closer to the heart’s grasp; because the negation of imperfection in One for Whom imperfection is impossible is an imperfection; because being occupied with this formula conveys to one the grandeur of the Truth through the negation of the alterities, since the negation of the alterities actually derives from the heart’s preoccupation with those very alterities. That is impossible for the person who is absorbed in the Light of Divine Unity.