Musnad Ahmad 1101:14

Yazid ibn Abd Rabbih narrated to us, Baqiyyah narrated to us, al-Walid ibn Kamil narrated to me, from al-Hajar, or Abu al-Hajar ibn al-Muhallab al-Bahrani, he said: Dubay'ah bint al-Miqdam ibn Ma'di Karib narrated to me, from her father: "That the Messenger of Allah, may Allah bless him and grant him peace, when he prayed facing a pillar, or a piece of wood, or something similar to that, he would not place it directly in front of his eyes, but rather he would place it to his left eyebrow."