Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) narrates: The Prophet (peace and blessings of Allah be upon him) entered his grave, that is, the grave of Sayyiduna Sa’d bin Mu’adh (may Allah be pleased with him). He stayed there for a little while. When he came out, it was asked, "O Messenger of Allah (peace and blessings of Allah be upon him), why did you stay there?" The Prophet (peace and blessings of Allah be upon him) said: "Sa’d was compressed in the grave. I supplicated to Allah Almighty, so He granted him relief."
Hadith Referenceصحیح ابن حبان / كتاب إخباره ﷺ عن مناقب الصحابة رجالهم ونسائهم بذكر أسمائهم رضوان الله عليهم أجمعين / 7034
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح لغيره - «الصحيحة» (4/ 270).فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده ضعيف