Musnad Ahmad 333:30

Hashim ibn al-Qasim narrated to us, he said: 'Ataf narrated to us, from Musa ibn Ibrahim ibn Abi Rabi'ah, he said: I heard Salamah ibn al-Akwa' say: I said: "O Messenger of Allah, I am out hunting and I pray while I have nothing on me except one shirt." He said: "Button it, even if you find nothing but a thorn."