Musannaf Ibn Abi Shayba 17:1350

Ubayd Allah ibn Musa narrated to us, he said: Shayban informed us, from Mansur, from Mujahid, from Sa'id ibn al-Musayyib, who said: The strap of Umar’s sandal broke, so he said: "Indeed we belong to Allah and indeed to Him we shall return." They said: "O Commander of the Faithful, is it [only] for the strap of your sandal?" He said: "Yes, everything that befalls the believer which he dislikes is a calamity."