Musannaf Ibn Abi Shayba 3:6412

Abu Bakr narrated to us, he said: Waki’ narrated to us, he said: Qurrah ibn Khalid al-Sadusi narrated to us, from Bakr ibn Abd Allah al-Muzani, that Umar sought permission [to enter] upon Abd al-Rahman ibn Awf while he was praying in his house. Umar said to him: "Shorten [your prayer]."