Sahih Bukhari Volume 2, Book 17, Hadith Number 141

Narated By 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).