Most of what we possess in this world is temporary except our soul, thus highlighting the importance of ensuring our soul is peaceful and serene as we live our everyday lives.
How do I know if my soul is tranquil? You want to know whether your nafs is 'nafs mut'ma'inna.
The first one is 'inshirah al-sadr' - a tranquil heart or soul with a big heart. What is a big heart? Check Surat Al Zumar, verse 22: "Afaman sharah Allah sadrahu li-l-Islam fahuwa alaa nurin min Rabbih."(39:22) A big heart simply means I enjoy Allah's worship.
When I come to Salah, I enjoy, I do not want the Salah to come to an end. When I do ma'tam, I enjoy because it is a form of 'Ibadah and I do not want it to come to an end. When I recite Qur'an I enjoy that means you have a tranquil heart. Salah, 'Ibadah lets the heart feel that warmth.
So if I want to know if I am 'nafsul-mutma'inna' and I want Allah to send the angels to address me before I depart from this world then during Salah I am okay.
Have you forgotten the munajaat? Imam Amir ul-Mu'mineen in his munaajat used to recite- "Kafaa bi 'izzan an takuna li Rabba, wa kafaa bi fakhran an akuna laka 'abdaa, anta kamaa uhibbu, faj'alni kamaa tuhibb." Allahu Akbar. Imam Ali says: 'It is enough honour for me that You are my Lord and I am proud that I Your servant. Oh Allah You are the way I want , make me the way You want.
That is the first sign of a tranquil soul. Some souls no, they are not tranquil. When he comes to 'Ibadah, he is so agitated, he want to finish. "Thumma qasat qulubukum min ba'di dhaalik fa hiya kal hijaarati aw ashaddu qaswah" (2:74). Qur'an says some of the souls are very hard, even harder than the stones and mountains.
"A'andhartahum am lam tundhirhum, la yu'minun." (2:6) You talk to them, you counsel them, you admonish them or you do not do that it is the same to them. So that is the first sign of having a tranquil soul.
The second sign is 'hudu' - calmness. Someone who has a tranquil soul is always calm, very calm and we refer back to the first verse, at Surat al Fath, verse 4, "anzala al-sakinata fi qulub al-mu'mineen" (48:4) Allah placed that tranquility and peace in the hearts, which makes them very relaxed. Big problem, he is relaxed because his aim is Allah and nobody else. Have we not forgotten Du'a 'Arafah of Abi Abdillah: 'what has he found who has lost You. And what has he lost who has found You. So the second sign is that tranquil heart is always calming, relax.
And the third sign of a tranquil soul is 'al thabaat,'- steadfastness. Do you see me, are we steadfast really? Do you think we are steadfast? We are not 'nafs al mutma'inna'. One of the manifestation of 'nafs al mutma'inna' is Aba 'Abdillah and if you claim to love Aba 'Abdillah then you must also be 'nafs al mutma'inna'. Thabaat! Allah in Qur'an says: "Yuthabbit Allahu al-ladhinaa aamanu bil qawli al-thabit fi al-hayaat al-dunyaa." (14:27) Those who truly believe in Allah, they are steadfast. Situation does not change them, problems do not change them. In fact, it makes them stronger because Imam Ali said Allah examines the one He loves the most. Hence He examined Ahlul Bayt more than any other.