Musnad Ahmad 31:522

Waki’ narrated to us, ‘Ubayd Allah ibn ‘Abd al-Rahman ibn Mawhab narrated to us, from his paternal uncle, from a freed slave of Abu Sa’id al-Khudri, that he was with Abu Sa’id while he was with the Messenger of Allah (peace and blessings of Allah be upon him). He said: The Prophet (peace and blessings of Allah be upon him) entered and saw a man sitting in the middle of the mosque, interlocking his fingers, talking to himself. The Prophet (peace and blessings of Allah be upon him) gestured to him, but he did not notice. He said: So he turned to Abu Sa’id and said: "When one of you prays, let him not interlock his fingers, for interlocking is from the Shaytan. For one of you remains in prayer as long as he is in the mosque, until he exits from it."