Invoking Allah for Rain (Istisqaa).

Bukhari :: Book 2 :: Volume 17 :: Hadith 140

Narrated 'Abdullah bin Zaid Al-Ansari:

The Prophet went out towards the Musalla in order to offer the Istisqa' prayer and when he intended to invoke (Allah) or started invoking, he faced the Qibla and turned his cloak inside out.

Bukhari :: Book 2 :: Volume 17 :: Hadith 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).

Bukhari :: Book 2 :: Volume 17 :: Hadith 142

Narrated Aisha:

Whenever Allah's Apostle saw the rain, he used to say, "O Allah! Let it be a strong fruitful rain."