The Messenger of Allah ﷺ used to raise his hands in supplication until I could see the whiteness of his armpits.Ubayy, who is Abu al-Mutamir, said, "I think this was only during the prayer for rain."
حدثنا عارم قال حدثنا معتمر قال وحدثني أبي عن بركة عن بشير بن نهيك
كان رسول الله ﷺ يرفع يديه في الدعاء حتى أرى بياض إبطيه قال أبي وهو أبو المعتمر لا أظنه إلا في الاستسقاء
Its isnad is sahih, Bukhari 8/724 no. 4958 narrated it with similar wording in Fath al-Bari in the tafsir of Surah Alaq, as did Muslim 4/2154 no. 2797 in the chapter on the description of the hypocrites, and Tirmidhi 5/443 no. 3348 in the tafsir of Surah Alaq.