Sahih Bukhari
Invoking Allah for Rain (Istisqaa)
Volume: 002 / Book Number: 017 / Hadith Number: 141
Narrated Anas bin Malik
The Prophet 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 raising his hands, but it is narrated that the Prophet used to raise his hands for invocations other than Istisqa. See Hadith No. 807 & 808 and also see Hadith No. 612, Vol. 5).