Sahih Bukhari

Narrated 'Ubai bin `Umar: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