Riyad us Saliheen: Hadith 1341

We accompanied the Prophet (ﷺ) in an expedition when he (ﷺ) said, "Some people have remained behind us in Al-Madinah, and we never cross a valley but they are with us. They share the reward with us because they have been held back by valid excuse."

In another narration the wordings are: "...by any genuine excuse."

In another narration the wordings are: "They are your partners in reward."

[Al- Bukhari].