Musannaf Ibn Abi Shayba 13:3084

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Shu’bah narrated to us, from ‘Asim ibn ‘Ubayd Allah, from ‘Abd Allah ibn ‘Amir ibn Rabi’ah, he said: I witnessed ‘Umar ibn al-Khattab make a false witness stand in the evening, while he was in an izar (waist-wrapper), poking himself.