I saw the Messenger of Allah ﷺ raising his hands (high enough) in supplication (for rain) that the whiteness of his armpits became visible.  

رَأَيْتُ رَسُولَ اللَّهِ ﷺ يَرْفَعُ يَدَيْهِ فِي الدُّعَاءِ حَتَّى يُرَى بَيَاضُ إِبْطَيْهِ  

Anas b. Malik reported that the Messenger of Allah ﷺ prayed for rain pointing the back of his hands to the sky.  

أَنَّ النَّبِيَّ ﷺ اسْتَسْقَى فَأَشَارَ بِظَهْرِ كَفَّيْهِ إِلَى السَّمَاءِ  

Anas reported that the Messenger of Allah ﷺ was not accustomed to raice his hands in any supplication he made except when praying for rain. (He would then raise [his hands] high enough) that the whiteness of his armpits became visible. 'Abd al-A'la said that (he was in doubt whether it was) the whiteness of his armpit or armpits.  

أَنَّ نَبِيَّ اللَّهِ ﷺ كَانَ لاَ يَرْفَعُ يَدَيْهِ فِي شَىْءٍ مِنْ دُعَائِهِ إِلاَّ فِي الاِسْتِسْقَاءِ حَتَّى يُرَى بَيَاضُ إِبْطَيْهِ غَيْرَ أَنَّ عَبْدَ الأَعْلَى قَالَ يُرَى بَيَاضُ إِبْطِهِ أَوْ بَيَاضُ إِبْطَيْهِ  

This hadith has been narrated by Anas b. Malik through another chain of transmitters.  

عَنِ النَّبِيِّ ﷺ نَحْوَهُ