Musannaf Ibn Abi Shayba 3:2447

Abu Bakr narrated to us, he said: Waki’ narrated to us, from Ubayd Allah ibn Abd al-Rahman ibn Wahb (Mawhab), from his paternal uncle, from a freed slave of Abu Sa’id al-Khudri, that he was with Abu Sa’id al-Khudri, and he was sitting with the Messenger of Allah (may Allah bless him and grant him peace). He said: The Prophet (may Allah bless him and grant him peace) entered the mosque and saw a man sitting in the middle of the mosque, interlacing his fingers, talking to himself. He said: The Prophet (may Allah bless him and grant him peace) gestured to him, but he did not notice. So he turned to Abu Sa’id al-Khudri and said: "If one of you prays, let him not interlace his fingers, for interlacing is from the devil. And one of you is still in prayer as long as he is in the mosque, until he exits it."