Sahih al-Bukhari 1031
Sahih al-Bukhari · Book 15: Invoking Allah for Rain (Istisqaa) · The raising of both hands by the Imam during Istisqa' while invoking Allah for rain
Narrated Anas bin Malik:
حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ ، حَدَّثَنَا يَحْيَى ، وَابْنُ أَبِي عَدِيٍّ ، عَنْ سَعِيدٍ ، عَنْ قَتَادَةَ ، عَنْ أَنَسِ بْنِ مَالِكٍ ، قَالَ كَانَ النَّبِيُّ صلى الله عليه وسلم لاَ يَرْفَعُ يَدَيْهِ فِي شَىْءٍ مِنْ دُعَائِهِ إِلاَّ فِي الاِسْتِسْقَاءِ، وَإِنَّهُ يَرْفَعُ حَتَّى يُرَى بَيَاضُ إِبْطَيْهِ.
The Prophet (ﷺ) 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 (ﷺ) raising his hands, but it is narrated that the Prophet (ﷺ) used to raise his hands for invocations other than Istisqa. See Hadith No. 807 & 808 and also see Hadith No. 612, Vol. 5).