Harmalah bin Imran al-Tujibi (may Allah have mercy on him) said that Ayyub bin Shurahbil (may Allah have mercy on him) wrote [a letter] to Umar bin Abd al-Aziz (may Allah have mercy on him), asking him about those who rob graves. Umar (may Allah have mercy on him) wrote to him: By my life! The one who steals from the dead is deemed to deserve the same punishment as the one who steals from the living.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب السرقة / 17242