Musnad Abu Ya'la al-Mawsili 115:70

Ibrahim ibn ‘Ar‘arah al-Sami narrated to us, Muslim ibn Qutaybah narrated to us, ‘Umar ibn Nabhan narrated to us, from Qatadah, from Anas: That the Messenger of Allah (ﷺ) used to supplicate with the insides of his palms, and he would say like this, displaying the backs of his palms.