Abd al-Malik bin Ata has reported this from Sayyiduna Abu Hurayrah (may Allah be pleased with him), with doubt on my part (as to whether) (the Prophet (peace and blessings be upon him) said it): "The one seeking permission is not to be given permission until he first greets with salam."
Tabarani recorded it in al-Mu'jam al-Awsat, and its narrators are thiqat (trustworthy), except that I have not found any report of the narrator Abd al-Malik having heard from Sayyiduna Abu Hurayrah (may Allah be pleased with him). Ibn Hibban says: He narrated from Yazid bin al-Asamm.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدب / 12744