Muhammad Ibn Ahmad Adh-Dhahabi writes:

أَلَا تَـرَى الـصَّـحَـابَـةَ مِـنْ فَــرْطِ حُـبِّـهِـمْ لِـلـنَّـبِـيِّ صَـلَّـى الـلَّـهُ عَـلَـيْـهِ وَسَـلَّـمَ قَـالُـوا أَلَا نَـسْـجُـدُ لَـكَ فَـقَـالَ لَا فَـلَـوْ أَذِنَ لَـهُـمْ لَـسَـجَـدُوا لَـهُ سُـجُـودَ إِجْـلَالٍ وَتَـوْقِـيـرٍ لَا سُـجُـودَ عِـبَـادَةٍ كَـمَـا قَـدْ سَـجَـدَ إِخْـوَةُ يُـوسُـفَ عَـلَـيْـهِ الـسَّـلَامُ لِـيُـوسُـفَ وَكَـذَلِـكَ الْـقَـوْلُ فِـي سُـجُـودِ الْـمُـسْـلِـمِ لِـقَـبَـرِ الـنَّـبِـيِّ صَـلَّـى الـلَّـهُ عَـلَـيْـهِ وَسَـلَّـمَ عَـلَـى سَـبِـيـلِ الـتَّـعْـظِـيـمِ وَالـتَّـبْـجِــيـلِ لَا يُـكَـفَّـرُ بِـهِ أَصْـلًا بَـلْ يَـكُـونُ عَـاصِـيًـا فَـلْـيَـعْـرِفْ أَنَّ هَـذَا مَـنْـهِـيٌّ عَـنْـهُ وَكَـذَلِـكَ الـصَّـلَاةُ إِلَـى الْـقَـبْـرِ

“Do you not see how the companions, out of love for the Prophet (s.), said: “Do we not prostrate ourselves before you?” Whereupon he said: “No.” However, if he had allowed them, they would have prostrated before him. A prostration out of respect and reverence. Not a prostration out of worship. Just as the brothers of Joseph prostrated themselves before Joseph (12:100) and the same assertion applies to a Muslim who prostrates himself to the tomb of the Prophet (s.) out of respect and reverence. He is basically not declared a disbeliever (kafir), but he would be a sinner. So he should know that it is forbidden for him to do so and likewise to pray to the grave.” [Mu’jam-ush-Shuyukh, page 56.]

Comment:

Prostration to graves is therefore not idolatry (Shirk) if the intention (Niyah) is to esteem and not worship the person.