Yahya related to me from Malik from Ibn Shihab that Ubaydullahibn Abdullah ibn Utba said, "I visited Umar ibn al-Khattab just beforenoon and found him praying a voluntary prayer. So I stood behind him,but he pulled me nearer and put me next to him, on his right handside, and then Yarfa came and I moved back and we formed a row behindhim."
USC-MSA web (English) reference: Book 9, Hadith 35
Arabic reference: Book 9, Hadith 364