Al-Lu'lu Wal-Marjan 516 — The Book of Salât Al-Istisqâ [i.e. to offer a two Rak'a prayer and then to invoke Allâh for rain at times of draught]

Anas bin Malik Narrated from:

The Prophet صلى الله عليه وسلم never raised his hands for any invocation except for that of Istisqâ '; and he used to raise them so much that the whiteness of his armpits became visible. ( Sahîh Al-Bukhâri, Hadîth No. 141, Vol. 2).