Hadith 779
Sayyiduna Zaid bin Aslam narrates that I saw Sayyiduna Ibn Umar (may Allah be pleased with them both) praying with his button open, so I asked him about this, and he replied that he had seen the Prophet Muhammad (peace and blessings be upon him) doing so.
Hadith 780
The Imam, through his esteemed teacher Muhammad bin Yahya’s chain of narration, reports from Walid in a manner similar to the above. However, he did not mention the words that I asked him and said that I saw the Messenger of Allah (peace be upon him) praying with his button undone.