“O children of Adam,
only by remembering and contemplating Me with your whole being are you truly faithful to Me. Whenever you forget Me, you are unfaithful.”
1. Forgetful of Pre-Existent Knowledge
It is a cardinal Sufi doctrine that the spirit or the soul bears the imprint of all knowledge from pre-existence, and, therefore, there is nothing for it to know that it does not already know. If the kingdom of God is within us, all knowledge is within us. But the drug of sensuality, the taint of worldliness, has made the spirit forgetful of its pre-existent knowledge. Purge it, and that knowledge will come back. Hence, it is said that knowledge is but recollection. ...
K. Khosla reading Jalâlu d-Dîn Rumi, Mathnawi II, p.168-77;
quoted in The Sufism of Rumi; K. Khosla
2. On the saying "He who knows himself ..."
As to that which as been related concerning the saying of one of the early mentors that, "He who knows himself, knows his Lord;"
it means that one cannot know one's Lord with knowledge of oneself. Thus when he forgets himself, he knows his Lord.
Sahl ibn `Abdallâh said, "The life of the heart is in the knowledge of God alone (not of another)."
Abu `Uthmân al-Hîrî and Abu Turâb al-Nakhshabî said, "One who is ignorant of God's decrees (ahkâm) cannot be one knowledgeable of God."
No one attains true knowledge of God while being ignorant of God's decrees and commands. How might one who has attained knowledge of God be ignorant of His decrees and commands?! When one knows his Lord, knows His decrees and commands, and lives by them to the best of his ability, he will manifest signs of sincerity and be counted among the sincere (sâdiqîn). Then, (in time) he will become firmly established in sincerity and will be subsequently be counted among the veracious (siddiqîn). ...
From the Stations Of The Righteous,
(Mas`alat darajât as-sâdiqîn fî -tasawwuf) by Abu `Abd ar-Rahmân as-Sulamî
3. On Knowing God - Truth
Shaykh Abû al-Hasan (al-Shâdhilî)
said, "How can the One through whom the gnostic sciences have been known be Himself known through the gnostic sciences? Or, how can the One whose existence preceded the existence of all things be known through any of these things?"
A disciple once asked his shaykh, "O Teacher where is God?" To which his shaykh replied, "May God crush you! After a vision [of the Divine] with your own eyes, you seek to know where he is?"
The Truth has thus been veiled from His servants by the magnificance of His appearance, while their sight has been prevented from witnessing Him by the very brilliance of His light.
Know that evidence is only provided for those who are seeking the truth, not for those who have witnessed it.
The Subtle Blessings in the Saintly Lives of Abul-Abbas al- Mursi And His Master Abû l-Hasan al- Shâdhilî (Kitab Lata'if al-Minan fi Manaqib Abi'l-Abbas al-Mursi wa Shaykhihi Abi'l-Hasan) by Ibn Ata' Allah al-Iskandari
4. Concerning the saying: "He who knows himself, knows his Lord."
Is (He who knows himself, knows his Lord)
a hadith of the Prophet?
Imam Sakhawi said in al-Maqasid al-Hasana:
"The hadith?: 'Whoever knows themselves knows their Lord.'
Abu al-Mudhaffar ibn al-Sam`ani said....,"It is not established as a Prophetic hadith. Rather, it is related from Yahya ibn al-Mu`adh al-Razi - that is, from his words."
This is also what Imam Nawawi said, affirming that, "It is not established." [Sakhawi, al-Maqasid al-Hasana 1.220]
Imam Saghani considered it a fabricated (mawdu`) hadith. [Saghani, al-Mawdu`at, 1.2] Other major works on fabricated hadiths state likewise.
Thus, this would not be permissible to ascribe to the Prophet peace and blessings be upon him. However, the words themselves are sound in meaning, and the great masters of the spiritual path quoted them extensively.
The meaning is also established by clear texts of the Qur'an and Sunna, such as the words of the Messenger of Allah peace and blessings be upon him, "Be mindful of Allah, and you'll find Him before you." [Tirmidhi, and others] 
And Allah alone gives success.
5 . On "Whoso Knows Himself ...",
attributed to Shaykh Muhyiddin Ibn `Arabi
The opening paragraphs from the book
'Whoso Knoweth Himself',
attributed to Shaykh Muhyiddin Ibn `Arabi:
In the name of God, the Merciful, the Compassionate,
and Him we ask for aid:
Praise be to
God [ Allah, may His Majesty be exalted ] before whose oneness there was not a before,
unless the Before were He, and after whose singleness
there is not an after, except the After be He. He is,
and there is with Him no after nor before, nor above
nor below, nor far nor near, nor union nor division,
nor how nor where nor when, nor times nor moment
nor age, nor being nor place. And He is now as He
was. He is the One without oneness, and the Single
without singleness. He is not composed of name and
named, for His name is He and His named is He.
So there is no name other than He, nor named.
And so He is the Name and the Named. He is the
First without firstness, and the Last without lastness.
He is the Outward without outwardness, and the
Inward without inwardness. I mean that He is the
very existence of the First and the very existence of
the Last, and the very existence of the Outward and
the very existence of the Inward. So that there is no
first nor last, nor outward nor inward, except Him,
without these becoming Him or His becoming them.
Understand, therefore, in order that you may
not fall into the error of the Hululis: He is not in a
thing nor a thing in Him, whether entering in or
proceeding forth. It is necessary that you know
Him after this fashion, not by knowledge (`ilm), nor
by intellect, nor by understanding, nor by imagination, nor by sense, nor by the outward eye, nor by
the inward eye, nor by perception. 'There' does not
see Him, save Himself; nor perceive Him, save
Himself. By Himself He sees Himself, and by
Himself He knows Himself. None sees Him other
than He, and none perceives Him other than He.
His Veil is (only a part of) His oneness; nothing
veils other than He. His veil is (only) the concealment of His existence in His oneness, without any
quality. None sees Him other than He - no sent
prophet, nor saint made perfect, nor angel brought
nigh knows Him. His Prophet is He, and His
sending is He, and His word is He. He sent Himself
with Himself to Himself. There was no mediator nor
any means other than He. There is no difference
between the Sender and the thing sent, and the
person sent and the person to whom he is sent. The
very existence of the prophetic message is His
existence. There is no other, and there is no
existence to other, than He, nor to its ceasing to be
(fana`), nor to its name, nor to its named.
"Whoso Knoweth Himself..."
Muhyiddin Ibn `Arabi
The Treatise on Being of Ibn `Arabi, also known as the Treatise on Unity. tnsl. by T.H. Weir. 27 pp
Soft cover temporarily out of print, also in
6. You do not know yourself ...
"Tell him, this is a long topic for explanation."
"You do not know yourself, you do not know your own attributes, nor where your soul is in its essence, nor where it travels. Or, where do your mind and your understanding go when you sleep? Ignoramus, you do not even know how the bread you just ate proceeds from you!"
Al-Suyuti in his Fatwa on the narration "Whoso knows himself..."
notes - hadith 1:
"Be mindful of Allah, and Allah will be mindful of you. Be mindful of Allah, and you'll find Him before you. And if you ask, ask Allah. If you rely, rely on Allah."
Related by Tirmidhi, who deemed it
sound and rigorously authentic (hasan sahih)