Musannaf Ibn Abi Shayba 3:2603

Abu Bakr narrated to us, he said: Yahya ibn Sa'id narrated to us, from 'Umar ibn Nabih, from Jumhan, who said: I saw Sa'd come repeatedly while the people were in prayer, so he walked between the row and the wall until he reached his place of prayer. And he used to pray by the fifth pillar.