Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) reports this saying of the Prophet (peace and blessings be upon him): "Whoever greets twenty Muslims with salam in a day, whether he greets them together or one by one, and then dies on that same day, Paradise becomes obligatory for him; and the same ruling applies to the night."
Tabarani recorded it. Its chain contains a narrator, Maslamah bin Ali, who is da'if (weak).
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الأدب / 12734