Musannaf Ibn Abi Shayba 3:5105

Waki’ narrated to us, he said: Thawr al-Shami narrated to us, from Rashid ibn Sa’d, from Malik ibn Yukhāmir—Thawr said once: Rashid ibn Sa’d or someone else—he said: I saw Mu’adh ibn Jabal killing lice and fleas while in prayer.