The Book of Praying for Rain (Al-Istisqa')
17/3. Chapter:
The recommended condition for the imam to be in if he goes out
Sunan an-Nasai
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.
Reference : Sunan an-Nasa'i 1507In-book reference : Book 17, Hadith 4English translation : Vol. 2, Book 17, Hadith 1508