Narrated Malik ibn Yasar as-Sakuni, al-Awfi: 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 bin Abd al-Hamid said: according to us Malik bin Yasar was a Companion of the Prophet ﷺ.
Hadith Referenceسنن ابي داود / كتاب تفريع أبواب الوتر / 1486
Hadith Gradingالألبانی:حسن صحيح | زبیر علی زئی:إسناده حسن, مشكوة المصابيح (2242), وللحديث شاھد انظر مجمع الزوائد (10/169)
1486. Commentary: In general supplications, only the palms should be raised. However, in the prayer for rain (Salat al-Istisqa), when supplicating for the removal of drought and dryness, as a gesture of optimism (tafa’ul), the backs of the hands should be turned upwards. This is established from the Sunnah of the Messenger of Allah (sallallahu alayhi wa sallam).
Source: Sunan Abu Dawood – Commentary by Shaykh Umar Farooq Saeedi, Page: 1486