(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.
Reference: Sahih al-Bukhari 1030
In-book reference: Book 15, Hadith 25
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 17, Hadith 140