Sunan Abi Dawud 1170
Narrated Anas:
The Prophet (peace be upon him) was not accustomed to raise his hands in any supplication he made except when praying for rain.  He would then raise them high enough so much so that the whiteness of his armpits was visible.
حَدَّثَنَا نَصْرُ بْنُ عَلِيٍّ، أَخْبَرَنَا يَزِيدُ بْنُ زُرَيْعٍ، حَدَّثَنَا سَعِيدٌ، عَنْ قَتَادَةَ، عَنْ أَنَسٍ، أَنَّ النَّبِيَّ صلى الله عليه وسلم كَانَ لاَ يَرْفَعُ يَدَيْهِ فِي شَىْءٍ مِنَ الدُّعَاءِ إِلاَّ فِي الاِسْتِسْقَاءِ فَإِنَّهُ كَانَ يَرْفَعُ يَدَيْهِ حَتَّى يُرَى بَيَاضُ إِبْطَيْهِ ‏.‏