Sahih Bukhari: Hadith 7081

Abu Musa asked permission to enter upon `Umar, but seeing that he was busy, he went away. `Umar then said, "Didn't I hear the voice of `Abdullah bin Qais? Allow him to come in." He was called in and `Umar said to him, "What made you do what you did." He replied, "We have been instructed thus by the Prophet" `Umar said, "Bring proof (witness) for this, other wise I will do so-and-so to you." Then `Abdullah bin Qais went to a gathering of the Ansar who then said, "None but the youngest of us will give the witness for it." So Abu Sa`id Al-Khudri got up and said, "We used to be instructed thus (by the Prophet)." `Umar said, "This tradition of the Prophet (ﷺ) remained hidden from me. Business in the market kept me busy."