We used to greet the Prophet ﷺ while he was praying and he used to answer our greetings. When we returned from An-Najashi (the ruler of Ethiopia), we greeted him, but he did not answer us (during the prayer) and (after finishing the prayer) he said, "In the prayer one is occupied (with a more serious matter)."  

كُنَّا نُسَلِّمُ عَلَى النَّبِيِّ ﷺ وَهُوَ فِي الصَّلاَةِ فَيَرُدُّ عَلَيْنَا فَلَمَّا رَجَعْنَا مِنْ عِنْدِ النَّجَاشِيِّ سَلَّمْنَا عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْنَا وَقَالَ إِنَّ فِي الصَّلاَةِ شُغْلاً  

I used to greet the Prophet ﷺ while he was in prayer and he would return my greeting, but when we returned (from Ethiopia) I greeted the Prophet (while he was praying) but he did not return the greeting, and (after finishing the prayer) he said, "In the prayer one is occupied (with a more serious matter)."  

كُنْتُ أُسَلِّمُ عَلَى النَّبِيِّ ﷺ وَهُوَ فِي الصَّلاَةِ فَيَرُدُّ عَلَىَّ فَلَمَّا رَجَعْنَا سَلَّمْتُ عَلَيْهِ فَلَمْ يَرُدَّ عَلَىَّ وَقَالَ إِنَّ فِي الصَّلاَةِ شُغْلاً  

We used to greet the Prophet ﷺ while he used to be in prayers, and he used to reply to our greetings. But when we came back from Najashi (the King of Ethiopia) we greeted him (while he was praying) and he did not reply to us. We said, "O Messenger of Allah ﷺ! We used to greet you in the past and you used to reply to us." He said, "Verily The Mind is occupied and busy with more important matter during the prayer." (So one cannot return One's greetings.)  

كُنَّا نُسَلِّمُ عَلَى النَّبِيِّ ﷺ وَهْوَ يُصَلِّي فَيَرُدُّ عَلَيْنَا فَلَمَّا رَجَعْنَا مِنْ عِنْدِ النَّجَاشِيِّ سَلَّمْنَا عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْنَا فَقُلْنَا يَا رَسُولَ اللَّهِ إِنَّا كُنَّا نُسَلِّمُ عَلَيْكَ فَتَرُدُّ عَلَيْنَا قَالَ إِنَّ فِي الصَّلاَةِ شُغْلاً فَقُلْتُ لإِبْرَاهِيمَ كَيْفَ تَصْنَعُ أَنْتَ قَالَ أَرُدُّ فِي نَفْسِي  

We used to greet the Messenger of Allah ﷺ while he was engaged in prayer and he would respond to our greeting. But when we returned from the Negus we greeted him and he did not respond to us; so we said: Messenger of Allah. we used to greet you when you were engaged in prayer and you would respond to us. He replied: Prayer demands whole attention.  

كُنَّا نُسَلِّمُ عَلَى رَسُولِ اللَّهِ ﷺ وَهُوَ فِي الصَّلاَةِ فَيَرُدُّ عَلَيْنَا فَلَمَّا رَجَعْنَا مِنْ عِنْدِ النَّجَاشِي سَلَّمْنَا عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْنَا فَقُلْنَا يَا رَسُولَ اللَّهِ كُنَّا نُسَلِّمُ عَلَيْكَ فِي الصَّلاَةِ فَتَرُدُّ عَلَيْنَا فَقَالَ إِنَّ فِي الصَّلاَةِ شُغُلاً  

We used to salute the Messenger of Allah ﷺ while he was engaged in prayer and he would respond to our salutation, but when we returned from the Negas, we saluted him and he did not respond to us. He said : Prayer demands one’s whole attention.  

كُنَّا نُسَلِّمُ عَلَى رَسُولِ اللَّهِ ﷺ وَهُوَ فِي الصَّلاَةِ فَيَرُدُّ عَلَيْنَا فَلَمَّا رَجَعْنَا مِنْ عِنْدِ النَّجَاشِيِّ سَلَّمْنَا عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْنَا وَقَالَ إِنَّ فِي الصَّلاَةِ لَشُغْلاً  

We used to greet the Messenger of Allah ﷺ while he was engaged in prayer and he would respond to our greeting. But when we returned from the Negus we greeted him and he did not respond to us; so we said: Messenger of Allah. we used to greet you when you were engaged in prayer and you would respond to us. He replied: Prayer demands whole attention. (Using translation from Muslim 538a)  

كُنَّا نُسَلِّمُ عَلَى رَسُولِ اللهِ ﷺ وَهُوَ فِي الصَّلَاةِ فَيَرُدُّ عَلَيْنَا فَلَمَّا رَجَعْنَا مِنْ عِنْدِ النَّجَاشِيِّ سَلَّمْنَا عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْنَا فَقُلْنَا يَا رَسُولَ اللهِ كُنَّا نُسَلِّمُ عَلَيْكَ فِي الصَّلَاةِفَتَرُدُّ عَلَيْنَا؟ فَقَالَ إِنَّ فِيَّ أَوْ فِي الصَّلَاةِ لَشُغْلًا  

We used to salute the Messenger of Allah ﷺ while he was engaged in prayer and he would respond to our salutation, but when we returned from the Negas, we saluted him and he did not respond to us. He said : Prayer demands one’s whole attention. (Using translation from Abū Dāʾūd 923)   

عَبْدُ اللهِ كُنَّا نُسَلِّمُ عَلَى النَّبِيِّ ﷺ فِي الصَّلَاةِ حَتَّى رَجَعْنَا مِنْ عِنْدِ النَّجَاشِيِّ فَسَلَّمْنَا عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْنَا وَقَالَ إِنَّ فِي الصَّلَاةِ شُغْلًا  

"كُنَّا نُسَلِّمُ عَلَى النَّبِىِّ ﷺ حَتَّى رَجَعْنَا مِنْ عِنْدِ النَّجَاشِىِّ فَسَلَّمْنَا عَلَيْهِ فَلَمْ يَرُدَّ عَلَيْنَا، وَقَالَ: إِنَّ في الصَّلَاةِ شُغْلًا".