(a) It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with him): "Whoever prays behind the imam, the recitation of the imam is sufficient for him."
(b) Through another chain it is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with him): "Whoever is praying with an imam, the recitation of the imam is sufficient for him."
(c) Abu al-Azhar said that Sayyiduna Abdullah bin Umar (may Allah be pleased with him) was asked about reciting behind the imam, and he said: "I feel shy before the Lord of this House (the Ka'bah) to offer a prayer in which I do not recite Surah al-Fatihah."
This is the authentic report from Ibn Umar as his own statement. The version narrated from Suwayd bin Sa'id from Ali bin Mushir from Ubaydullah as marfu' (attributed to the Prophet) is an error; Suwayd's memory changed towards the end of his life, so errors multiplied in his narrations. It was also narrated from Kharijah bin Mus'ab from Ayyub from Nafi' as marfu', and Kharijah is not used as evidence. Abdan bin Muhammad al-Hafiz al-Marwazi said: The hadith of Kharijah from Ayyub from Nafi' from Ibn Umar from the Prophet (peace and blessings be upon him), "Whoever has an imam", is a munkar (rejected) error; it is only Ibn Umar's own statement, although the opposite of it has also been narrated from Ibn Umar.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2901