Sayyiduna Anas (may Allah be pleased with him) says that the Messenger of Allah (peace and blessings be upon him) used to come to the house of Umm Sulaym (may Allah be pleased with her) and sleep on her bed while she was not there. Then he said: One day she was called and told: "Here is the Messenger of Allah (peace and blessings be upon him) on your bed." She came to him, and he (peace and blessings be upon him) had sweated profusely, and this was in the hot season. She took a small bottle and began collecting the sweat and filling the bottle with it. The Messenger of Allah (peace and blessings be upon him) woke up and he (peace and blessings be upon him) said: "What will you do with this?" She said: "O Messenger of Allah! It is your blessing; we will put it in our perfume." The Messenger of Allah (peace and blessings be upon him) said: "You have done right."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1198
It is narrated from Nafi' that Sayyiduna Abdullah bin Umar (may Allah be pleased with him) would perform ablution in the heat and pass his hands over his armpits, and this did not break his ablution.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 1199