Sahih Al Bukhari Hadith Number 1030

The narrator Anas added that the Prophet (peace be upon him) raised his hands (during the invocation) to such an extent that the whiteness of his armpits was visible.

15  Invoking Allah for Rain (Istisqaa)
Hadith number 1030
Volume no 2
Chapter: 21 (While offering the Istisqa’ prayer, people should raise hands along with the Imam)