صحيح مسلم

'Ubaid b. Umair reported that Abu Musa brought permission from Umar (to enter the house) three times, and finding him busy came back, whereupon Umar said (to the Inmates of his house): Did you not hear the voice of 'Abdullah b. Qais (the Kunya of Abu Musa Ash'ari)? He was called back. and he (Hadrat 'Umar) said: What did prompt you to do it? Thereupon, he said: This is how we have been commanded to act. He (Hadrat 'Umar) said: Bring evidence (in support of) it, otherwise I shall deal (strictly) with you. So he (Abu Musa) set out and came to the meeting of the Ansar and asked them to bear witness before hadrat Umar about this. They (the Companions present there) said: None but the youngest amongst us would bear out this fact. So Abu Sa'id Khudri (who was the youngest one in that company) said: We have been commanded to do so (while visiting the house of other people). Thereupon 'Umar said: This command of Allah's Messenger (ﷺ) had remained hidden from me up till now due to (my) business in the market.