Musnad Ahmad 31:395

Muhammad ibn Abdullah ibn al-Zubayr narrated to us, he said: Ubaydullah ibn Abd al-Rahman ibn Mawhab narrated to us, he said: My paternal uncle, meaning Ubaydullah ibn Abdullah ibn Mawhab, narrated to me, from a freed slave of Abu Sa’id al-Khudri, who said: While I was with Abu Sa’id al-Khudri, with the Messenger of Allah (peace and blessings of Allah be upon him), we entered the mosque. There was a man sitting in the middle of the mosque, sitting with his knees drawn up to his chest (muhtabiyan), with his fingers interlaced with one another. The Messenger of Allah (peace and blessings of Allah be upon him) gestured to him, but the man did not notice the gesture of the Messenger of Allah (peace and blessings of Allah be upon him). He (the Prophet) turned to Abu Sa’id and said: "When one of you is in the mosque, let him not interlace his fingers, for interlacing is from the Shaytan. And one of you remains in a state of prayer as long as he is in the mosque, until he exits it."