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.
Sahih al-Bukhari Book 15, Hadith 25
← Previous Hadith A bedouin came to Allah's Messenger (ﷺ) on a Friday and said, "O Allah's Messenger ! The livestock, the offspring, and the people have perished." So, Allah's Messenger (ﷺ) raised both his hands invoking Allah (for rain) and the people too raised their hands with Allah's Messenger (ﷺ) invoking Allah (for rain). We had not left the mosque when it started raining. It rained till the next Friday when the same man came to Allah's Messenger (ﷺ) and said, "O Allah's Messenger! The travelers are compelled to postpone their journeys (because of excessive rain) and the roads are overflowed."

Sahih al-Bukhari Book 15, Hadith 24 Next Hadith → The Prophet (ﷺ) 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 (ﷺ) raising his hands, but it is narrated that the Prophet (ﷺ) used to raise his hands for invocations other than Istisqa. See Hadith No. 807 & 808 and also see Hadith No. 612, Vol. 5).

Sahih al-Bukhari Book 15, Hadith 26