Musannaf Ibn Abi Shayba 3:269

Hashim ibn al-Qasim narrated to us, from ‘Ikrimah ibn ‘Ammar, he said: ‘Asim ibn Shumaykh al-Ghaylani, one of the Banu Tamim, narrated to me, he said: I entered upon Abu Sa’id and I saw him while he was prostrating, keeping his elbows away from his sides until I saw the whiteness of his armpits.