Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) says: When the imam gets stuck (in his recitation), do not prompt him, for that is speech.
Al-Tabarani recorded it in al-Mu'jam al-Kabir, and its narrators are the narrators of the Sahih.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب الصلاة / 2352