Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) relates: When Sayyiduna Sa'd bin Mu'adh (may Allah be pleased with him) was buried and we were with the Messenger of Allah (peace and blessings be upon him), he (peace and blessings be upon him) said Subhan Allah, and the people too glorified Allah with him for a long time; then he (peace and blessings be upon him) said Allahu Akbar, and the people too said the takbir with him. The people said: O Messenger of Allah! For what reason did you glorify Allah? He (peace and blessings be upon him) said: "The grave of this righteous man had become narrow upon him, until Allah widened it by His mercy."
Hadith Referenceالمعجم الكبير للطبراني / باب السين / 5327