Sufyan bin Abd al-Malik (may Allah have mercy on him) said: I heard Abdullah bin al-Mubarak (may Allah have mercy on him) say: In my view, the hadith of Abdullah bin Mas'ud (may Allah be pleased with him) that "the Prophet (peace and blessings be upon him) raised his hands only the first time and then did not do so again" is not established, whereas the hadith of raising the hands is established with me; Ubaydullah, Malik, Ma'mar and Ibn Abi Hafsah narrated it from al-Zuhri, he from Salim, from Ibn Umar (may Allah be pleased with them both), and Ibn Umar (may Allah be pleased with them both) narrates it from the Prophet (peace and blessings be upon him); and I consider there to be latitude in it. Then Abdullah bin al-Mubarak (may Allah have mercy on him) said: Because of the abundance of the hadiths and the soundness of their chains, this is my view; it is as though I am looking at the Prophet (peace and blessings be upon him) raising his hands in prayer.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 2533