Praise be to Allaah.
It
is mustahabb to pray naafil prayers at all times, except at the times
when praying is not allowed. These times are 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 sun has set, except in specific cases, 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 when it 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, or that might involve any
resemblance to the mushrikeen. With regard to 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 should refrain from
praying at these times.
Summarized
from the fatwas of Shaykh Ibn ‘Uthaymeen, 1/354