This may have two meanings. If I sit and perform salaah Sayyidina Rasulullah Sallallahu'Alayhi Wasallam will remain standing alone. This was said to be a bad action and a sign of disrespect. There is a possibility that by saying 'sit down, he meant ending his prayer. It is clear that this is rude, bad and shows disrespect and carelessness, that while performing salaah with Sayyidina Rasulullah Sallallahu 'Alayhi Wasallam to end the niyyah and sit down. While standing he became tired, then this type of thoughts entered his mind.