Musnad Ahmad 509:12

Sulayman ibn Harb narrated to us, Hammad narrated to us, from ‘Asim ibn Bahdalah, from Habib ibn Abi Jubayrah, from Ya‘la ibn Sayabah, that the Prophet, may Allah bless him and grant him peace, passed by a grave and said: "Indeed, the occupant of this grave is being punished, and it is not for a major [sin]." Then he called for a palm branch and placed it on his grave, and said: "Perhaps it will be lightened for him as long as it remains moist."