مِشْكَاةُ الْمَصَابِيحِ
Mishkat al-Masabih 1498
Prayer
Prayer for Rain - Section 1
وَعَنْ أَنَسٍ قَالَ: كَانَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ لَا يَرْفَعُ يَدَيْهِ فِي شَيْءٍ مِنْ دُعَائِهِ إِلَّا فِي الِاسْتِسْقَاءِ فَإِنَّهُ يَرْفَعُ حَتَّى يرى بَيَاض إبطَيْهِ
Anas said the Prophet (ﷺ) was not accustomed to raise his hands in any supplication he made except when praying for rain. He would then raise them high enough for the whiteness under his armpits to be visible. (Bukhari and Muslim.)
Mishkat al-Masabih, 1498مُتَّفَقٌ عَلَيْهِ (الألباني)