From Sahih Muslim (Hadith 1445):
A'isha reported:The Apostle of Allah (ﷺ) one night delayed (observing the 'Isya' prayer) till a great part of the night was over and the people in the mosque had gone to sleep. He (the Holy Prophet) then came out and observed prayer and said: This is the proper time for it; were it not that I would impose a burden on my people (I would normally pray at this time). In the hadith transmitters by 'Abd al-Razzaq (the words are):" Were it not that it would impose burden on my people
Reference: Muslim 1445