Musnad Ahmad 25:47

'Abd al-A'la narrated to us, from Ma'mar, from al-Zuhri, from 'Ubayd Allah ibn 'Abd Allah, from 'Abd Allah ibn 'Abbas and from 'A'ishah, that they both said: When the Messenger of Allah (may Allah bless him and grant him peace) was overcome [by the agony of death], he began to cast a khamisa (a garment) over his face. When he felt distressed, we would remove it from him. He was saying: "May Allah curse the Jews and the Christians; they took the graves of their prophets as places of worship." 'A'ishah said: He was warning them against what they had done.