بسم الله الرحمن الرحيم
و الصلاة و السلام على أشرف المرسلين
و على اله و اصحابه أجمعين

In arabic:

The word sirr refers to something which is covered or hidden and which can not be understood. Its holder try hard to keep it secret through his words and his gestures. As for the word sarirah: in arabic, it means what the person keeps secret.
[al Mu’jam al ‘arabi al asasi].

In Tariqa Karkariya:

The sirr refers to the softness of the Divine Mercy which is immaculate, without stain, coming from whorship aspiration and put in the deepest and the most hidden inner of the heart. The fruit of this Mercy softness is the vision of the Oneness breath flowing in the primordial theophany of His Essence (handful of the Light, al Qabda Nuraniya).

In the Quran:

The word sirr (secret) is mentioned two times in the Qu’ran:
“And if you speak aloud, then indeed, He knows the secret and what is [even] more hidden” [surah Ta Ha, verse 7].

“Say: “It has been revealed by He who knows [every] secret wwithin the heavens and the earth. Indeed, He is ever Forgiving and Merciful.”
[surah al furqan, verse 6].

In the Hadith:

Abu Huraira (radi Allah anhu) said:
“I memorised from the Messenger of Allahﷺ two vessels (of knowledge). As for one of them, I have disclosed it, but as for the other, if I were to disclose it, this throat would be cut.”
[sahih Bukhari, n°118]

Abdullah bin ‘Umar (May Allah be pleased with them) reported:
“My father ‘Umar (May Allah be pleased with him) said: “When (his daughter) Hafsah (May Allah be pleased with her) became a widow, I met ‘Uthman bin ‘Affan (May Allah be pleased with him) and offered Hafsah for marriage to him”. ‘Uthman said: “I shall think over the matter.” I waited for a few days and then ‘Uthman met me and said: “It occurred to me that I should not marry at present.” Then I met Abu Bakr (May Allah be pleased with him) and said to him: “If you are willing, I shall marry my daughter Hafsah to you.” Abu Bakr (May Allah be pleased with him) remained silent and did not utter any word to me in reply. I grew more angry with him than with ‘Uthman. I had waited for only a few days when Messenger of Allah ﷺ asked for her hand in marriage and I married her to him. Thereafter, I met Abu Bakr (May Allah be pleased with him) who said: “Perhaps you were angry with me when you offered Hafsah to me and I said nothing in reply.“ I said: “Yes, that is so.” He said: “Nothing stopped me to respond to your offer except that I knew that Messenger of Allah ﷺ had mentioned her and I could not disclose the secret of Messenger of Allah ﷺ. Had Messenger of Allah ﷺ left her, I would have accepted her.””
[Sahih Bukhari, N°3731].

Ibn ‘Umar narrates the Messenger of Allah ﷺ said: ”Do not talk about [divine] Destiny because it is the Secret of Allah. Do not reveal the Secret of Allah.”
[Huliyat ul Awliyah, ibn Na’im, hadith 8396].

Abdullah ibn Mas’ud reported:
“The Messenger of Allah ﷺ said: “Allah will gather during a chosen day the first and the last generations. They will remain there standing for forty years, looking [tormented] at the sky waiting the Judgement to be delivered. Allah will come down from the Throne clouds to the Kursi and a call will be heard: “ O men, do you accept from your Lord who created you and who gave you goods and who ordered you to whorship Him and to not associate Him, to follow what each of you have worshipped in the past life? Is it not Justice coming from your God? They will say: “yes, it is”. Each community will go towards its idol. All the idols they worshipped will be presented to them. Some of them will follow the sun, others will follow the moon and others the stones. Those who worshipped Jesus, the demon who was assigned to Jesus will be presented to them. Those who worshipped ‘Uzayr, the demon who was assigned to ‘Uzayr will be presented to them. Muhammadﷺ and His community will wait. Allah will present himself to them and tell them: “why did you not go away like the others?” and they will say: “We have a god we have not seen yet” and He will say: “would you recognize him if you see Him?” they will say: “there is a SIGN between He and us so we can recognize Him””.
[Reported by at-Tabarâni]

Abu ‘Umama reported :
“The Prophet ﷺ: “The man I am the most satisfied is the faithful servant, who is penniless but who tastes his act of worship, who obeys his Lord and who perfects secretly his acts of worship. Who is not seen among people and who is not showed by people, he is content with only the bare necessities, and in this state he is patient. His death is quick and gentle, only few people mourn his death and his inheritance is consistent””.
[Musnad Ahmad ibn Hanbal, Hadîth 21617].