We used to greet the Messenger of Allah ﷺ while he was in prayer, and he would return our greeting. When we returned from the court of the Negus, we greeted him but he did not return our greeting. He said, "In prayer one is occupied."
كنا نسلم على رسول الله ﷺ وهو في الصلاة فيرد علينا. فلما رجعنا من عند النجاشي سلمنا عليه فلم يرد علينا وقال: إن فى الصلاة شغلا
Bukhari narrated it in the Book of Actions in Prayer (1199), and Muslim in the Book of Mosques (538), both from Ibn Numayr, who said, Ibn Fudayl narrated to us, Amash narrated to us, from Ibrahim, from Alqama, from Abdullah, who related the like of it. Abu Dawud (924) and Nasai (1221) also transmitted it, and Ibn Hibban graded it sahih (2243), all of them through routes from Asim, from Abu Waail, from Abdullah, who said, "We used to give the greeting of peace during prayer and give instructions about our needs. Then I came to the Messenger of Allah ﷺ and greeted him, but he did not return the greeting, and I was overcome by both old and new distress. When the Messenger of Allah ﷺ finished the prayer, he said, 'Allah brings about of His command whatever He wills, and Allah, Mighty and Majestic, has now brought about of His command that you must not speak during prayer.' Then he returned my greeting."Its chain of transmission is hasan because of Asim, who is Ibn Abu Najud, spelled with a nun and jeem, the Asadi, their freedman, Abu Bakr the reciter, "truthful, but had lapses, an authority in recitation, and his hadith in the Sahihayn is only cited in combination with another narrator." Abu Waail is Shaqiq bin Salama al-Asadi. Bukhari mentioned it in definitive form from Ibn Masud in Fath al-Bari (13/496).Nasai also transmitted it (1220) with another chain, through the route of Sufyan, from Zubayd bin Adi, from Kulthum, from Abdullah, and he added in it, "that you must not speak except in remembrance of Allah, and it is not fitting for you to do otherwise, and that you should stand before Allah in humble devotion." Its chain of transmission is sahih. Kulthum is Ibn Alqama bin Najiya bin Mustaliq, and he is reliable. There is another chain for it, but it contains a narrator who has not been established as reliable.As for his statement, "both old and new distress," it means grief and gloom. He means that his old sorrows had returned to him and had connected with fresh ones. This is what Khattabi said.