The Prophet ﷺ prayed for rain and pointed with the backs of his hands toward the sky.In another wording, when the Messenger of Allah ﷺ supplicated, he would turn the backs of his hands toward his face and the palms toward the ground.
أن النبي ﷺ استسقى فأشار بظهر كفيه إلى السماء. وفي لفظ: أن رسول الله ﷺ لو كان إذا دعا، جعل ظاهر كفيه مما يلي وجهه، وباطنهما مما يلي الأرض.
Muslim narrated it in the chapter on the prayer for rain (896:6), from Abd bin Humayd, who said, Hasan bin Musa (that is, Ibn Ashyab) told us, Hammad bin Salama told us, from Thabit, from Anas, and he mentioned it.The other wording was narrated by Ahmad (12239), from Yazid (Ibn Harun), who said, Hammad bin Salama informed us with it.As for what Abu Dawud narrated (1487) through the route of Umar bin Nabhan, from Qatada, from Anas, who said, "I saw the Messenger of Allah ﷺ supplicating like this, with the inner sides of his palms and their outer sides," this is not sound. For Umar bin Nabhan is unanimously agreed to be daif, and Ibn Main's statement about him varies. Aqeeli mentioned him in al-Duafa (3/193) and quoted from Bukhari his statement, "Umar bin Nabhan narrated from Qatada, and he is not corroborated in his hadith," referring to this hadith of his. That is why Aqeeli recorded it for him and then said, "It has been narrated from the Prophet ﷺ through a route other than this one, with a chain sounder than this." End of quote.A note, there is no contradiction between this hadith and the earlier hadith of Malik bin Yasar, because the hadith of Anas concerns the supplication for rain specifically, while the hadith of Malik bin Yasar concerns supplication in general. Nawawi said in his commentary on Muslim (6/190), "The sunnah in every supplication made to lift a calamity, such as drought and the like, is to raise the hands with the backs of the palms turned toward the sky, while in a supplication asking for and obtaining something, the palms are turned toward the sky." End of quote.And Hafiz said in al-Fath (2/518), "It has been said that the wisdom behind turning the backs of the palms upward in the prayer for rain, unlike in other supplications, is to draw a good omen from the reversal of circumstances, turning from back to front, just as has been said regarding the turning of the cloak."