Musannaf Ibn Abi Shaybah 3516 — A man praying with his buttons undone

Sayyiduna Salamah bin al-Akwa' (may Allah be pleased with him) said to the Prophet (peace and blessings be upon him): "O Messenger of Allah! I am a man who hunts; may I pray in a single shirt?" He said: "Yes, but fasten it, even if only with a thorn."