Musnad Ahmad 980:10

Waki’ narrated to us, Sufyan narrated to us, from Salamah, from ‘Iyad ibn ‘Iyad, from his father, from Abu Mas’ud, who said: The Messenger of Allah (peace and blessings of Allah be upon him) delivered a sermon to us. He praised Allah and extolled Him, then said: "Indeed, there are hypocrites among you, so whoever I name, let him stand up." Then he said: "Stand up, O so-and-so. Stand up, O so-and-so. Stand up, O so-and-so," until he had named thirty-six men. Then he said: "Indeed, there are [hypocrites] among you, or from you, so fear Allah." He said: Then ‘Umar passed by a man from among those who were named, who was covering his face, and he [‘Umar] knew him. He said: "What is the matter with you?" He said: So he told him what the Messenger of Allah (peace and blessings of Allah be upon him) had said. He [‘Umar] said: "Away with you for the rest of the day."