I saw the Messenger of Allah (ﷺ) supplicating Allah in this manner with the palms of his hands and also with their backs upwards.
Sunan Abi Dawud Book 8, Hadith 72
← Previous Hadith The Prophet (ﷺ) said: When you make requests to Allah, do so with the palms of your hands, and not backs, upwards.

Abu Dawud said: The narrator Sulaiman b. 'Abd al-Hamid said: according to us Malik b. Yasar was a Companion of the Prophet (ﷺ).

Sunan Abi Dawud Book 8, Hadith 71 Next Hadith → The Prophet (ﷺ) said: Your Lord is munificent and generous, and is ashamed to turn away empty the hands of His servant when he raises them to Him.

Sunan Abi Dawud Book 8, Hadith 73