صحيح البخاري Sahih Bukhari كتاب الاستسقاء The Book of Al-Istisqa.
بَابُ رَفْعِ الإِمَامِ يَدَهُ فِي الاِسْتِسْقَاءِ:

(22) Chapter. The raising of both hands by the Imam during Istisqa while invoking Allah for rain.

1 hadith
Hadith 1031
حَدَّثَنَا مُحَمَّدُ بْنُ بَشَّارٍ ، حَدَّثَنَا يَحْيَى ، وَابْنُ أَبِي عَدِيٍّ ، عَنْ سَعِيدٍ ، عَنْ قَتَادَةَ ، عَنْ أَنَسِ بْنِ مَالِكٍ ، قَالَ : " كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ لَا يَرْفَعُ يَدَيْهِ فِي شَيْءٍ مِنْ دُعَائِهِ إِلَّا فِي الِاسْتِسْقَاءِ ، وَإِنَّهُ يَرْفَعُ حَتَّى يُرَى بَيَاضُ إِبْطَيْهِ " .
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).
Hadith Reference Sahih Bukhari / The Book of Al-Istisqa. / Hadith: 1031
Hadith Grading محدثین: أحاديث صحيح البخاريّ كلّها صحيحة