[Avodah] Tefillas Mincha Voluntary?

[Zev Sero]

Tal Moshe Zwecker wrote:
> Shalom uBracha All,
> 
> I am looking for a source that Mincha (the Tefillah not the offering) is 
> voluntary and not a chiyuv. I know we say this regarding Maariv that it 
> is reshus. I am asking because of the following Kedushas Levi:
> 
> 
> Kedushas Levi to (Bereishis 24:63)
> 
> [...]
> We are obligated to pray the shacharis prayer because Hashem returns our 
> souls to us [in the morning], and in these prayers we say [before the 
> Shemoneh Esrei] the prayer of Emes V’Yatziv (Hashem is true and 
> steadfast), and we thank Hashem for all His many favors, such as the 
> shining sun.
> 
> We are obligated to pray the evening prayer because we are now giving 
> over our soul to Hashem for safekeeping as a trust [when we sleep], and 
> we believe that the Blessed Creator is faithful and trustworthy to 
> return this soul with which He was entrusted, and therefore we say Emes 
> VeEmunah (Hashem is true and faithful).
> 
> However we are not obligated to pray minchah. It is voluntary, and 
> therefore it is called “minchah,” which means “gift” or nedavah."

Having seen it inside (thanks to R David Roth) it's clear that he's not
talking about halachic obligation but about the moral obligation that a
person has to thank and praise and pray to Hashem when He does something
for him.  It's obvious that a person ought to thank Hashem in the morning
when he has received his soul back, the sun has risen, and he has been
given a whole new day.  And it's also obvious that when a person is
about to deposit his soul with Hashem for the night, he ought to declare
his trust in Hashem and pray to Him.  But in the middle of the day, it's
not obvious what should trigger a sudden urge to pray.  What has Hashem
done for a person just at that moment?  Mincha is not in return for
anything in particular, it's "just because", and so it's called a gift.

The fact that the midday "gift" is halachically required, whereas the
evening obligation is not, i.e. a person is halachically permitted to
be a kefuy tovah in the evening and not thank Hashem for the day,
doesn't change this.  (Cf the other thread about morals.)

-- 
Zev Sero                      The trouble with socialism is that you
zev at sero.name                 eventually run out of other people’s money
                                                     - Margaret Thatcher