Sahih al-Bukhari, Book 15, Hadith 25 (In-Book) ⓘ
The narrator Anas added that the Prophet raised his hands (during the invocation) to such an extent that the whiteness of his armpits was visible.
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.
also referenced under: Hadith 1030 (DarusSalam) & Volume 2, Book 17, Hadith 140 (USC-MSA)