مُوَطَّأُ مَالِكٍ
Muwatta Malik 410
Shortening the Prayer
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، مَرَّ عَلَى رَجُلٍ وَهُوَ يُصَلِّي فَسَلَّمَ عَلَيْهِ فَرَدَّ الرَّجُلُ كَلاَمًا فَرَجَعَ إِلَيْهِ عَبْدُ اللَّهِ بْنُ عُمَرَ فَقَالَ لَهُ إِذَا سُلِّمَ عَلَى أَحَدِكُمْ وَهُوَ يُصَلِّي فَلاَ يَتَكَلَّمْ وَلْيُشِرْ بِيَدِهِ .
Yahya related to me from Malik from Nafi that Abdullah ibn Umar passed by a man who was praying and said, "Peace be upon you," and the man replied to him. Abdullah ibn Umar returned to him and said, "When someone says, 'Peace be upon you' to you while you are praying do not reply, but give a signal with your hand."
Arabic/English book reference: Book 9, Hadith 79