Musnad Ahmad 2933 — Musnad of Abdullah ibn al-Abbas ibn Abd

Shu’bah (may Allah have mercy on him) says that a man came to Sayyiduna Ibn Abbas (may Allah be pleased with them both) and said, “Your freed slave, while prostrating, places his forehead, arms, and chest on the ground.” He asked him the reason for this, so he mentioned humility as the cause. He replied, “This is how a dog sits. I have seen that when the Prophet (peace and blessings be upon him) prostrated, the whiteness of his blessed armpits would be visible (meaning his arms were so separated).”