Defining the time for salaat al-istisqaa’ (prayerfor rain)

Is there any report from the Prophet (peace and blessings of Allaah be
upon him) which definesthe time for going out topray Salaat
al-Istisqaa' (prayer for rain)?
Praise be to Allaah.
There is a hadeeth in Sunan Abi Dawood with an acceptable isnaad from
'Aaishah (may Allaah be pleased with her) which states that the
Prophet (peace and blessings of Allaah be upon him) set a day when the
people should go out. 'Aa'ishah said: "The Messenger of Allaah(peace
and blessings of Allaah be upon him) went out when the rim of the sun
appeared…" The hadeeth clearly indicates that a day was set when the
people were to go out to pray Salaat al-Istisqa', even though it does
not namethis day. More than one of the scholars have stated that it is
mustahabb to choose Mondays and Thursdays, because deeds are shown to
Allaah on those days, and because these are good days for fasting. So
if the Muslimscombine fasting and praying for rain, their du'aa's in
this case are more likely to be accepted.
It may be said that it is not prescribed to single out these two days
rather than others, because that was not proven from the Prophet(peace
and blessings of Allaah be upon him) or from any of the Sahaabah, and
this is thecorrect view. So it is not prescribed to choose one day
rather than another without there being any text to prove that. What
is prescribed is to choose a day on which the people shouldgo out,
which may happen to be a Monday or any other day, according to what is
in the people's best interests and according to what is convenient for
them.
Shaykh Sulaymaan ibn Naasir al-'Alwaan