Musnad Ahmad 333:28

Hammad ibn Khalid narrated to us, he said: Attaf ibn Khalid narrated to us, from Musa ibn Ibrahim, from Salamah ibn al-Akwa’, who said: I said to the Prophet, peace and blessings of Allah be upon him: "I am sometimes out hunting, and I pray in my shirt." He said: "Button it, even if you find nothing but a thorn."