I saw the Prophet ﷺ praying for rain, and he extended his hands with their backs facing the sky.
حدثنا مؤمل حدثنا حماد حدثنا ثابت
رأيت النبي ﷺ يستسقي فبسط يديه ظاهرهما مما يلي السماء
No references are available.
Its isnad is sahih. We have already mentioned that supplication for desiring something is made with the palms of the hands facing upward, while supplication for seeking protection from something is made with the backs of the hands facing upward. This is also found in Muslim, 2/612, no. 896, in the chapter on rain-seeking, raising the hands in supplication. See also 11035.