It is narrated from al-Hasan (may Allah have mercy on him) that he saw no harm in a person remembering Allah within himself (during the sermon) with takbir, tahlil and tasbih.
Mansur bin al-Mu'tamir (may Allah have mercy on him) asked Ibrahim (may Allah have mercy on him): Do you recite while the imam is delivering the sermon on Friday? He said: Perhaps it will not harm you.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الجمعة / 5836