Mishkat al-Masabih 4 Prayer كتاب الصلاة 432

وَعَنْ نَافِعٍ قَالَ: إِنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ مَرَّ عَلَى رَجُلٍ وَهُوَ يُصَلِّي فَسَلَّمَ عَلَيْهِ فَرَدَّ الرَّجُلُ كَلَامًا فَرَجَعَ إِلَيْهِ عَبْدُ اللَّهِ بْنُ عُمَرَ فَقَالَ لَهُ: إِذَا سُلِّمَ عَلَى أَحَدِكُمْ وَهُوَ يُصَلِّي فَلَا يَتَكَلَّمْ وَلْيُشِرْ بِيَدِهِ. رَوَاهُ مَالك

Nafi’ said that ‘Abdallah b. ‘Umar passed a man while he was praying and gave him a salutation to which the man uttered some words in reply. ‘Abdallah b. ‘Umar went back to him and said to him, “When one of you is greeted while he is engaged in prayer he must not speak, but give a sign with his hand.” Malik transmitted it.