Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates: I heard the Prophet (peace and blessings be upon him) say: "When a person enters the mosque while the imam is on the pulpit, then there is to be no prayer and no talking until the imam has finished."
Al-Tabarani recorded it in al-Mu'jam al-Kabir; its chain contains Ayyub bin Nahik, who is matruk (abandoned); a group declared him da'if (weak). Ibn Hibban mentioned him in al-Thiqat and said that he makes mistakes.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 3120