Musannaf Ibn Abi Shayba 3:366

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Hasan ibn Salih, from Musa ibn Abi ‘Aisha, from ‘Abd al-Rahman ibn Abi ‘Asim, from Abu Hind al-Shami, he said: ‘Umar said: "When one of you prostrates, let him place his palms directly upon the ground, so that perhaps Allah may avert from him the shackles, should he be shackled on the Day of Resurrection."