Nafi' (may Allah have mercy on him) narrated: A man from the people of Egypt told me that he prayed the morning prayer with Umar bin al-Khattab (may Allah be pleased with him) at al-Jabiyah. He recited Surah al-Hajj and made two prostrations in it. Nafi' (may Allah have mercy on him) said: When he concluded with the salam, he said: This surah has been given superiority because it contains two prostrations. And Sayyiduna Ibn Umar (may Allah be pleased with them both) also used to make two prostrations in it.
Although this report from Umar, through Nafi', is in effect mursal, since Nafi' did not name the Egyptian who narrated it to him, the first report, from Abdullah bin Tha'labah bin Su'ayr from Umar, is a sahih (authentic), connected (mawsul) report; and likewise Nafi''s report from Ibn Umar is connected.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3730