I saw the Messenger of Allah ﷺ supplicating Allah in this manner with the palms of his hands and also with their backs upwards.

رَأَيْتُ رَسُولَ اللَّهِ ﷺ يَدْعُو هَكَذَا بِبَاطِنِ كَفَّيْهِ وَظَاهِرِهِمَا