Sahih al-Bukhari 1030

Arabic

وَقَالَ الأُوَيْسِيُّ حَدَّثَنِي مُحَمَّدُ بْنُ جَعْفَرٍ، عَنْ يَحْيَى بْنِ سَعِيدٍ، وَشَرِيكٍ، سَمِعَا أَنَسًا، عَنِ النَّبِيِّ صلى الله عليه وسلم أَنَّهُ رَفَعَ يَدَيْهِ حَتَّى رَأَيْتُ بَيَاضَ إِبْطَيْهِ‏.‏

English translation

The narrator Anas added that the Prophet (ﷺ) raised his hands (during the invocation) to such an extent that the whiteness of his armpits was visible

Reference: Sahih al-Bukhari 1030, Book of Invoking Allah for Rain (Istisqaa). English translation by Dr. Muhammad Muhsin Khan.