سُنَنُ النَّسَائِيِّ
Sunan an-Nasa’i 1507
The Book of Praying for Rain (Al-Istisqa')
The recommended condition for the imam to be in if he goes out
أَخْبَرَنَا قُتَيْبَةُ، قَالَ حَدَّثَنَا عَبْدُ الْعَزِيزِ، عَنْ عُمَارَةَ بْنِ غَزِيَّةَ، عَنْ عَبَّادِ بْنِ تَمِيمٍ، عَنْ عَبْدِ اللَّهِ بْنِ زَيْدٍ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم اسْتَسْقَى وَعَلَيْهِ خَمِيصَةٌ سَوْدَاءُ .
It was narrated from Abdullah bin Zaid that: The Messenger of Allah (ﷺ) prayed for rain wearing a black khamisah.
Sunan an-Nasa'i, 1507Sahih (Darussalam)