Sunan Ibn Majah Book 5, Hadith 160
“The Messenger of Allah (ﷺ)
said: ‘I
have gained weight, so when I bow, then bow, and when I stand
up,
then stand up, and when I prostrate, then prostrate. I should
never
find anyone preceding me in bowing or prostration.’”
“The Messenger of Allah (ﷺ) said: ‘Does not the one who raises his head before the Imam fear that Allah may turn his head into the head of a donkey?’”
“The Messenger of Allah (ﷺ) said: ‘Do not bow or prostrate before me. No matter how far ahead of you I bow, you will catch up with me when I stand up, and no matter how far ahead of you I prostrate, you will catch up with me when I raise my head. I have become bulky.’”