Know - may God lead you on the way of realization and lead you, and us, on the road to success! - that there is no way to travel the Sufi path (tasawwuf), especially if one is aspiring toward unveiling (kashf) and realization (tahqīq), without constant and complete obedience and submission to a "realized" (muhaqqiq) shaykh, a spiritual guide (murshid) joining together esoteric truth (haqīqa) and exoteric law (sharī`a). For the spiritual path is perilous and the slightest deviation from the road that is laid out results in missing the goal by a great distance.

Abū l-Hasan al-Shushtarī 1 expressed it this way:
"He (that is, the aspirant) cannot do without placing his affairs in the hands of someone who will command him [to do good], forbid him [what is evil], and watch over him. For the way is perilous: few are the travellers and many are those who cut short the road.
Often, the traveller believes himself to be in the middle of the road when he is already turning his back on his goal. It has sufficed that he deviate by the distance of a finger, and he is already off the path and lost.
The way is narrow, in truth, for him whose soul, following anchored habits, wishes to dispose freely of a body (badan) that it is called to leave. The demon of this road knows well its stations and stages."

Abū Yazīd is reported to have said the following:
"He who, has no master has Satan as his master."

Al-Daqqāq 2 said:
"The tree that grows by itself, without having been planted, grows leaves but no fruit. If it does grow fruit its fruit is not like the fruit of trees that have been planted.

Abū `Amr al-Zajjājī 3 said:
"Nothing will come from the man who has no master, even should he have visions of the invisible."

Shaykh Abū l-`Abbās al-Mursī 4 said:
"He who has not had a shaykh on this path [the Sufi path] will not be able to make his neighbor happy. Even were he endowed with great intelligence and a submissive soul, such a man, if he is satisfied with what the master of erudition (shayh al-ta`līm) teaches him, he will not reach the perfection of him who places himself in the hands of a master educator (shayh al-murabbī).

The soul is always covered with thick veils, strongly inclined to associate [illusory realities with God], and it is inevitably left with the stains of futility (ru`ūnāt). This state cannot totally cease unless one places oneself in the hands of a third person, and if one places oneself under the person's authority and power.
Such is also the case for those for whom God has reserved special graces, whom He has taken and attracted in His Presence: those individuals will not attain the function of spiritual guide (mashyaha), despite how far they have progressed."
...

notes:

1 Abū l-Hasan al-Shushtarī , an Andalusian traveller on the way and poet

2 Abū 'Alī al-Daqqāq, d. 412 /1021, was al-Qushayri's Sufi master, as well as his father-in-law.

3 Abū `Amr al-Zajjājī, A Sufi from Nishapour; in Baghdad he was the disciple of Junayd, and spent a number of years in Mecca, where he died in 348 / 959-960.

4 Schaykh Abū l-`Abbās al-Mursī, successor to Imam Shādhilī, d. 694/1296 in Alexandria. One of his disciple was Ibn `Atā' Allāh.