Riyad as-Salihin

Buraidah (May Allah be pleased with him) reported: Messenger of Allah (ﷺ) said, "(In the past) I forbade you from visiting graves, but visit them now."

In another narration Messenger of Allah (ﷺ) said, "Whoever wants to visit a graveyard, let him do so, because it reminds us of the Hereafter."

[Muslim].