Musannaf Ibn Abi Shayba 3:1813

'Abd al-Salam ibn Harb narrated to us, from 'Ata' ibn al-Sa'ib, from Abu 'Abd al-Rahman al-Sulami, who said: We used to recite to Abu 'Abd al-Rahman while we were walking. When he would pass by a prostration (sajdah), he would say "Allahu Akbar," gesture, and say "al-salam." And he claimed that Ibn Mas'ud used to do that.