Language:
كتاب الاستسقاء
15
Invoking Allah for Rain (Istisqaa)
(21)
Chapter: While offering the Istisqa' prayer, people should raise hands along with the Imam
(21)
باب رَفْعِ النَّاسِ أَيْدِيَهُمْ مَعَ الإِمَامِ فِي الاِسْتِسْقَاءِ
The narrator Anas added that the Prophet (ﷺ) raised his hands (during the invocation) to such an extent that the whiteness of his armpits was visible.

وَقَالَ الأُوَيْسِيُّ حَدَّثَنِي مُحَمَّدُ بْنُ جَعْفَرٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، وَشَرِيكٍ، سَمِعَا أَنَسًا، عَنِ النَّبِيِّ صلى الله عليه وسلم أَنَّهُ رَفَعَ يَدَيْهِ حَتَّى رَأَيْتُ بَيَاضَ إِبْطَيْهِ‏.‏
Reference : Sahih al-Bukhari 1030
In-book reference : Book 15, Hadith 25
USC-MSA web (English) reference : Vol. 2, Book 17, Hadith 140
  (deprecated numbering scheme)