Sahih Al Bukhari Hadith Number 1031

Narrated Anas bin Malik: The Prophet (peace be upon him) never raised his hands for any invocation except for that of Istisqa’ and he used to raise them so much that the whiteness of his armpits became visible. (Note: It may be that Anas did not see the Prophet (peace be upon him) raising his hands, but it is narrated that the Prophet (peace be upon him) used to raise his hands for invocations other than Istisqa. See Hadith No. 807 & 808 and also see Hadith No. 612, Vol. 5).

15  Invoking Allah for Rain (Istisqaa)
Hadith number 1031
Volume no 2
Chapter: 22 ( The raising of both hands by the Imam during Istisqa’ while invoking Allah for rain)