Musannaf Ibn Abi Shayba 21:541

Humaid ibn Abd al-Rahman narrated to us, from Sa'id ibn al-Sa'ib, from Dawud ibn Abi 'Asim, who said: I stood with Salim ibn Abd Allah at Arafat, watching what he would do. He remained in dhikr (remembrance of Allah) and supplication until he departed.