Musannaf Ibn Abi Shaybah

Hazrat Abu Saeed narrates that Hazrat Abu Musa Ash'ari sought permission from Hazrat Umar three times, but he did not grant him permission, so he returned. Hazrat Umar sent a messenger to call him and asked, "What made you return?" He replied, "I sought permission three times, as the Messenger of Allah (peace and blessings be upon him) commanded us: if we are granted permission, we may enter, and if not, we should return." Hazrat Umar said, "Bring evidence for this, otherwise I will do such and such (I will punish you)." So he went to a gathering of people and, making them swear, asked about this matter. They testified in his favor, then Hazrat Umar let him go.