We used to greet the Messenger of Allah ﷺ with the salam while he was praying, and he would return our greeting. When we returned from our visit to Najashi, we greeted him but he did not return our greeting.We said, "Messenger of Allah, we used to greet you during prayer and you would return our greeting."He replied, "Indeed in prayer there is occupation enough."
حدثنا محمد بن فضيل حدثنا الأعمش عن إبراهيم عن علقمة
كنا نسلم على رسول الله ﷺ وهو في الصلاة فيرد علينا فلما رجعنا من عند النجاشي سلمنا عليه فلم يرد علينا فقلنا يا رسول الله كنا نسلم عليك في الصلاة فترد علينا فقال إن في أو في الصلاة لشغلا
Its isnad is sahih. Alqama is Ibn Qays bin Abdullah Nakhai, brother of Abdul-Rahman, and maternal uncle of Ibrahim bin Yazid. He is a senior, trustworthy tabii, born during the lifetime of the Messenger of Allah ﷺ, and he is one of the people most knowledgeable about Ibn Masud. The hadith was narrated by the two Shaykhs, as noted in al-Muntaqa 1061.