Sahih Muslim · Hadith 1235
The Book of Mosques and Places of Prayer - كتاب الْمَسَاجِدِ وَمَوَاضِعِ الصَّلاَةِ
حدثنا أبو كامل الجحدري، حدثنا حماد بن زيد، عن كثير، عن عطاء، عن جابر، قال كنا مع النبي صلى الله عليه وسلم فبعثني في حاجة فرجعت وهو يصلي على راحلته ووجهه على غير القبلة فسلمت عليه فلم يرد على فلما انصرف قال " إنه لم يمنعني أن أرد عليك إلا أني كنت أصلي " .
Jabir reported: We were in the company of the Messenger of Allah (may peace be upon him), and he sent me on an errand, and when I came back (I saw him) saying prayer on his ride and his face was not turned towards Qibla. I greeted him but he did not respond to me. As he completed the prayer, he said: Nothing prevented me from responding to your greeting but the fact that I was praying.
Gradings are classical jarḥ-wa-taʿdīl, name-matched to a rijāl index (not individually re-verified).