The Messenger of Allah ﷺ prayed for rain while wearing a black cloak.
أخبرنا قتيبة قال حدثنا عبد العزيز عن عمارة بن غزية عن عباد بن تميم
أن رسول الله ﷺ استسقى وعليه خميصة سوداء
Its isnad is sahih. Abu Dawud narrated it in "Prayer, Chapter on the collection of chapters on the prayer for rain and its subdivisions," no. 1164, from Qutayba with this wording, and it is also in Kubra, no. 1809. Ibn Mulaqqin graded it sahih in Tuhfat al-Muhtaj, no. 734, as did Hakim, who said it meets the standard of Muslim, 1/327, and Dhahabi agreed with him.