Hadith: The Messenger of Allah (may Allah's peace and blessings be upon him) was once riding a horse and fell down from it, injuring his right side

عن أنس بن مالك أن رسول الله صلى الله عليه وسلم ركب فرسا، فصُرِع عنه فجُحِش شِقُّه الأيمن، فصلى صلاة من الصلوات وهو قاعد، فصلَّينا وراءه قعودا، فلما انصرف قال: إنما جُعِل الإمام ليُؤتمَّ به، فإذا صلى قائما، فصلوا قياما، فإذا ركع، فاركعوا وإذا رفع، فارفعوا، وإذا قال: سمع الله لمن حمده، فقولوا: ربنا ولك الحمد، وإذا صلى قائما، فصلوا قياما، وإذا صلى جالسا، فصلوا جلوسا أجمعون.
[صحيح] - [رواه البخاري]

Anas ibn Mālik (may Allah be pleased with him) reported that the Messenger of Allah (may Allah's peace and blessings be upon him) was once riding a horse and fell down from it, injuring his right side. He led one of the prayers while sitting, and we prayed behind him sitting as well. When he finished the prayer, he said: "The Imām is meant to be followed, so when he prays standing, then pray standing; and when he bows, bow; and when he rises from bowing, rise; and when he says: 'Allah hears those who praise Him)', then say: 'Our Lord, for You is all praise'; and when he prays standing, then pray standing; and when he prays sitting, then all of you pray sitting."
Sahih/Authentic. - [Al-Bukhari]

Explanation

The Prophet (may Allah's peace and blessings be upon him) was riding a horse and fell down from it, injuring his right side. So he led the Companions in one of the prayers while he was sitting. In turn, they prayed behind him while they were sitting. When he finished the prayer, the Prophet (may Allah's peace and blessings be upon him) informed them that the congregation follows the Imām in everything. When he says "Allah is the Greatest," they say it as well; when he bows, they bow; when he prostrates, they prostrate; if he prays standing, then they should pray standing like him; and if he prays sitting, then they should pray sitting like him. This is so if he starts the prayer in the sitting posture and is a regular Imām. And this is what happened with the Companions (may Allah be pleased with him) when the Prophet (may Allah's peace and blessings be upon him) fell from the back of his riding mount one day and his right side was affected, and so he prayed while sitting, and the Companions prayed behind him while sitting too.