Musannaf Ibn Abi Shayba 3:2294

Waki’ narrated to us, from Sufyan, from Abu Ishaq, from ‘Amr ibn Maymun, who said: When ‘Umar was stabbed and the people became agitated, ‘Abd al-Rahman ibn ‘Awf stepped forward and recited the two shortest surahs in the Qur’an: "Indeed, We have granted you, [O Muhammad], the Abundance" (Al-Kawthar) and "When the victory of Allah has come and the conquest" (An-Nasr).