Is it proven that the Prophet (blessings and peace of Allah be upon him) ever sat on a chair?
Question

Is there any report which says that the Prophet (blessings and peace of Allah be upon him) sat on a chair?
Praise be to Allah.The Prophet (blessings and peace of Allah be upon him) was extremely humble towards people, and showed true servitude to Allah, the Lord of the Worlds. Ahmad (7160) narrated that Abu Hurayrah said: Jibril sat before the Prophet (blessings and peace of Allah be upon him). He looked up at the heavens and saw an angel descending. Jibril said: This angel has never descended since the day he was created until this hour. When the angel had descended, he said: O Muhammad, your Lord has sent me to you [to ask you]: Should He make you a king-prophet, or a slave-messenger? Jibril said: Be humble before your Lord, O Muhammad. He said: “Rather a slave-messenger.”
The commentators on Musnad Ahmad said: Its isnad is sahih according to the conditions of al-Bukhari and Muslim.
He used to sit on the ground, and eat from [a cloth placed on] the ground.
At-Tabarani narrated in al-Kabir (12494) that Ibn ‘Abbas said: The Messenger of Allah (blessings and peace of Allah be upon him) used to sit on the ground and eat from [a cloth placed on] the ground. He would take hold of the sheep and milk it, and he would accept the invitation of a slave to a meal of barley bread. Classed as sahih by al-Albani in as-Sahihah (2125).
Al-Baghawi narrated in Sharh as-Sunnah (11/287) that ‘A’ishah said: I said: O Messenger of Allah, may Allah cause me to be sacrificed for you, eat reclining, for it will be easier for you. He lowered his head until his forehead was almost touching the ground, and said: “No; rather I shall eat as a slave eats and sit as a slave sits.” Classed as sahih by al-Albani in as-Sahihah (544).
Ibn al-Qayyim (may Allah have mercy on him) said:
The Prophet (blessings and peace of Allah be upon him) used to sit directly on the ground sometimes, and on a reed mat sometimes, and on a rug sometimes."(Ighathat al-Lahfan  1/126).
This is how he (blessings and peace of Allah be upon him) was most of the time.
But sometimes he needed to make the people hear his words and convey his message to all of them, so he would call for a chair and sit on it, and teach the people.
Muslim (876) narrated that Abu Rifa‘ah said: I came to the Prophet (blessings and peace of Allah be upon him) when he was delivering a khutbah and I said: O Messenger of Allah, here is a stranger who has come to ask you about his religion, for he does not know what his religion is. The Messenger of Allah (blessings and peace of Allah be upon him) turned to me and left his khutbah. He came to me, and a chair was brought; I thought its legs were made of iron. The Messenger of Allah (blessings and peace of Allah be upon him) sat on it and started telling me of what Allah had told him, then he went back and completed his khutbah.
An-Nawawi (may Allah have mercy on him) said:
He (blessings and peace of Allah be upon him) sat on the chair so that the others could hear his words and see him. End quote.
It is proven that his Companions (may Allah be pleased with them) also sat on chairs and couches.
Al-Bukhari (1594) narrated that Abu Wa’il said: I sat with Shaybah on a chair in the Ka‘bah. He said: ‘Umar (may Allah be pleased with him) sat in this place. Shaybah was the son of Ibn ‘Uthman ibn Abi Talhah (a Companion), may Allah be pleased with him.
Abu Dawud (113) narrated that ‘Abd Khayr said: I saw ‘Ali (may Allah be pleased with him) when a chair was brought to him and he sat on it… Classed as sahih by al-Albani in Sahih Abi Dawud.
Al-Bukhari (7266) narrated that Abu Jamrah said: Ibn ‘Abbas used to seat me on his couch.
So there is nothing wrong with sitting on a chair. If a person needs to sit on it in order to teach people and make them hear him, this is something recommended, as it is proven that the Prophet (blessings and peace of Allah be upon him) did that.
And Allah knows best.