Sayyiduna Abu Sa'id al-Khudri (may Allah be pleased with him) says that Sayyiduna Abu Musa al-Ash'ari (may Allah be pleased with him) sought permission three times to come to Umar (may Allah be pleased with him), then went back. Umar (may Allah be pleased with him) asked: "Why did you not come to me?" He replied: "I heard the Prophet (peace and blessings be upon him) say: 'Whoever seeks permission should seek it three times; if he receives no answer, he should go back.'" Umar (may Allah be pleased with him) said: "Produce a witness for this, or I will punish you." Sayyiduna Abu Musa (may Allah be pleased with him) became alarmed and spoke to the people, and Sayyiduna Ubayy bin Ka'b (may Allah be pleased with him) said: "The youngest man among the people will bear witness for you." In those days I, Abu Sa'id, was the youngest among them, so I went and testified before Umar (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "Whoever seeks permission three times and is not given permission should go back."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأشربة والحد فيها / 17664