The Prophet ﷺ prayed for rain, raising his arms until I saw, or until the whiteness of his armpits was seen. Mutamir said, "I think this was during the prayer for rain."
حدثنا أبو بكر بن أبي شيبة حدثنا عفان حدثنا معتمر عن أبيه عن بركة عن بشير بن نهيك
أن النبي ﷺ استسقى حتى رأيت أو رئي بياض إبطيه قال معتمر أراه في الاستسقاء
[Its chain of transmission is sahih] Ahmad narrated it: 2/370, from the hadith of Mutamir, and Ibn Abu Adi followed him in narrating it, on pages 235 and 236, and Busiri said, "This is a sahih chain, its narrators are trustworthy." Majashi, Abu Walid, is trustworthy, as stated in Taqrib and other sources. Barakah.