Dought - clear - Ruling on praying at times when prayer is notallowed.

One of my friend, who isvery punctual in offering Namaaz, has seen
offering namaaz sometimes even when the sun is setting. He is of the
view that offeringNamaaz at sun set or sunrise time is not a sin and
it might be just a makrooh. I told him that offering prayer at sun set
or sun rise time is notallowed and it might be a sin also. Because to
have distinction with non-muslims, it has not been allowed to offer
namaaz at this time.
Please tell me whether offering of prayers)Namaaz( at sun set or sun
rise time is makrooh or a sin and why?
Praise be to Allaah.
It is mustahabb to pray naafil prayers at all times, except at the
times when praying is not allowed. These timesare from after Fajr
prayer until the sun has risen to the height of a spear; from noon
time until after the sun has passed its zenith, which is at mid-day
for about five minutes before the sun reaches its zenith; and from
after 'Asr prayer until the sun sets. What is meant is the prayer
which each person does by himself; once he has prayed 'Asr it becomes
haraam for him to pray until the sunhas set, except in specificcases,
when it is not haraam. See Question # 306.
The reason why it )prayer at these times( is not allowed is so as to
avoid any resemblance to the kuffaar who prostrate to the sun when it
rises as a greeting to it and to express their joy, and they prostrate
to it whenit sets as a farewell to it. The Prophet )peace and
blessings of Allaah be upon him( was keen to put a stop to all things
that may lead to shirk, orthat might involve any resemblance to the
mushrikeen. With regardto the prohibition on praying when the sun is
near its zenith, this is because this is the time when the Fire of
Hell is intensified, as was reported from the Prophet )peace and
blessings of Allaah be upon him(, so we shouldrefrain from praying at
these times.