This excerpt is taken from Sheikh Abi Abdul Saboor Abdul Ghafoor Damni's book Fazail al-Jihad.
عن أنس بن مالك رضى الله عنه أن رسول الله صلى الله عليه وسلم رجع من غزوة تبوك فدنا من المدينة فقال إن بالمدينة أقواما ما سرتم مسيرا ولا قطعتم واديا إلا كانوا معكم قالوا يا رسول الله وهم بالمدينة قال وهم بالمدينة حبسهم العذر
"Anas bin Malik (may Allah be pleased with him) narrated that the Messenger of Allah (peace be upon him) returned from the Battle of Tabuk. When he reached near Madinah Munawwarah, he said: 'There are people in Madinah who, despite being in Madinah, remained with you in the journey and valleys.' The Companions (may Allah be pleased with them) said: 'O Messenger of Allah! Were they present in Madinah?' He said: 'Yes, they were present in Madinah; an excuse prevented them from participating in the battle.'"
Reference: (Sahih Bukhari)
Reference: (Narrated by Bukhari: Book of Military Expeditions, Chapter (81), Number: 4423)
Explanation
The Prophet Muhammad (peace be upon him) spoke about those sincere people who wished to participate in the battle wholeheartedly but were prevented by an excuse. Some had physical constraints, and some had financial difficulties. Due to their sincerity, Allah Almighty granted them the reward along with those who participated in the battle.
