halacha - Source for Chasidik practice of imagining yourself giving up your life during the first verse of Krias Shma

in a chassidus shiur i heard that the obligation for krias shma (for people busy with parnoso) according to all opinions is the first posuk of shma with kavono

and that the kavono is by saying it you are excepting the yoke of the kinship of haven (i guess the source of this is the mishna)
and that you are ready to give your live for echod (Hashem)
what is the source of this?

i thought that bicol nafshicha means this but what does it have to do the first verse and with the word "ehod"?

Answer

The first source is multiple, a mishna, a berayta with Rabbi Akiva, Rishonim.

1. Mishna Berachoth 9, 5 (pasted here from the Gemara with Soncino's translation)"
   

   
   

   'WITH ALL THY SOUL' MEANS, EVEN THOUGH HE TAKES THY SOUL [LIFE]"

   
   

   
   but the verse cited is the second verse and not the first.


2. Gemara Berachoth 61B
   

   
   

   When R`Akiba was taken. He prolonged the word ehad until he expired while saying it. A bath kol went forth and proclaimed: Happy art thou, Akiba, that thy soul has departed with the word "ehad"!

   
   see here

   
   It seems this is the first verse that is in question.
   
   The explanation is that Rabbi Akiva was amazed by the verse. He wondered, how is it possible to love G_d even when they kill us; that is to say, when it ceases to manifest his presence by the protection it has on us. The lack of perception of G_d is a sign that we are facing something far greater than previously believed. This means that we discover our blindness across as what it really is, and it is much more than what we were we represented (This is an idea often found in the books of Chassiduth, for example Rabbi Menachem Mendel Mivitebsk in Pri Ets, Drush Leparshath Teshuva If you understand this you may link what he says with this Gemara and an other Gemara with Rabbi Akiva that treat about midath hadin.). One day Rabbi Akiva was able to do all the effort he has developed this Cavana out of the Kriath Shema and in the general rapport with the existence of G_d. He is Echad, in all situations. It is in this spirit that we see a hint in several books as it has in the Rosh's Orchoth Chayim in Rishonim.


3. Here is Orchoth Chayim from HaRosh (his source is Igeret Hateshuva from Rabenu Yona). He extends this thought to the love of God in general. not just krias shema:
   

   
   

   לאהוב את ה' אלהיך בכל לבבך וגו', למסור גופך וממונך על קדושתו, ובזה תקיים בעצמך דברי המשורר "כי עליך הורגנו כל היום". לבטוח בה' בכל לבבך, ולהאמין בהשגחתו הפרטית, ובזה תקיים בלבבך הייחוד השלם בהאמין בו, כי עיניו משוטטות בכל הארץ, ועיניו על כל דרכי איש, ובוחן לב וחוקר כליות. כי מי שאינו מאמין "אשר הוצאתיך מארץ מצרים", אף ב"אנכי ה' אלוהיך" אינו מאמין, ואין זה ייחוד שלם. כי זה היה סגולת ישראל על הגויים, וזה יסוד כל התורה כולה:

   
   

   
   


4. Noam Elimelech, Tsetel Koton in the 2 first paragraphs developp the general idea and the cavana in KS. . The first is a general consideration, the second on krias shema. Here is it in wikitext.

I hope this may help you in this wonderful question.

And there is an English translation of the Tsetel Koton: