Musnad Ahmad 788:26

'Abd al-Razzaq narrated to us, Ma'mar informed us, from Sa'id al-Jurayri, from Abu Nadra, from Abu Sa'id al-Khudri, who said: 'Abdullah ibn Qays, Abu Musa al-Ash'ari, greeted 'Umar ibn al-Khattab—may Allah be pleased with them both—three times, but he was not given permission [to enter], so he returned. 'Umar sent [someone] after him, [asking]: "Why did you return?" He said: "I heard the Messenger of Allah, peace and blessings of Allah be upon him, say: 'If one of you greets three times and is not answered, then let him return.'"