(a) Al-Aswad and Amr bin Maymun (may Allah have mercy on them both) say: We prayed Fajr behind Sayyiduna Umar (may Allah be pleased with him), and he did not recite the qunut.
(b) In the chapter on qunut we have already narrated from the Messenger of Allah (peace and blessings be upon him) and from the Rightly Guided Caliphs (may Allah be pleased with them) that they recited the qunut in the morning prayer. It is reported with sound chains from Sayyiduna Umar (may Allah be pleased with him) that he used to recite the qunut in the morning prayer. If these people omitted the qunut on some occasions, whether through forgetfulness or deliberately, this indicates that it is not obligatory; and since it is not reported from any one of them that they performed the prostrations of forgetfulness, this indicates that forgetting the qunut does not require the prostration of forgetfulness. And Allah knows best.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 3879