From Sahih Bukhari (Hadith 5815):
Narrated `Aisha and `Abdullah bin `Abbas:When the disease of Allah's Messenger (ﷺ) got aggravated, he covered his face with a Khamisa, but when he became short of breath, he would remove it from his face and say, "It is like that! May Allah curse the Jews Christians because they took the graves of their prophets as places of worship." By that he warned his follower of imitating them, by doing that which they did
Reference: Bukhari 5815