What he asked not to happen to his own grave
He gave the reason immediately after.
اللَّهُمَّ لاَ تَجْعَلْ قَبْرِي وَثَنًا يُعْبَدُ اشْتَدَّ غَضَبُ اللَّهِ عَلَى قَوْمٍ اتَّخَذُوا قُبُورَ أَنْبِيَائِهِمْ مَسَاجِدَ
Allāhumma lā tajʿal qabrī wathanan yuʿbad. Ishtadda ghaḍabu-Llāhi ʿalā qawmin-ittakhadhū qubūra anbiyāʾihim masājid
O Allah, do not make my grave an idol that is worshipped. The anger of Allah grew severe against a people who took the graves of their prophets as places of worship.
Where this comes from
He was praying against something that would be done in his own honour, by people who loved him, after he was gone.
The full narration
Yahya related to me from Malik from Zayd ibn Aslam from Ata ibn Yasar that the Messenger of Allah, may Allah bless him and grant him peace, said, "O Allah! Do not make my grave an idol that is worshipped. The anger on those who took the graves of their Prophets as places of prostration was terrible
Source
Muwaṭṭa' Mālik · 416
- Salim al-Hilali: Sahih Lighairihi