Musannaf Ibn Abi Shayba 3:5517

Waki’ narrated to us, he said: Sharik narrated to us, from Abu Ishaq, from Abu al-Ahwas, that Ibn Mas’ud came to Abu Musa in his house, and the prayer time arrived. Abu Musa said to Abdullah, "Step forward." Abdullah said, "You are more entitled." So Abu Musa stepped forward and took off his sandals. Abdullah said to him, "Are you in the Sacred Valley?"